It's time for entry number 2. I'd like to thank Duke of Luns for keeping the thread warm.
There's no fancy speech, no clever moral here. Just pure and simple Alt mode awesomesauce.
#2: Cybertronian Jets/SpacecraftLet's make it happen.

"Taxi!"
The first figure on this list is an unusual one. Jetfire, AKA Skyfire AKA Fluffyfire was one of the few early Autobot aircraft, one of the few defectors and in the comics one of the few Transformers to be built solely on Earth. Depending on which continuity you choose to believe he's either an old college buddy of Starscream's who's been stuck in the deep freeze next to the vegetable crisper, a drone built by Shockwave or an aloof Autobot scientist. Whichever continuity it is he's a serious, secular sort.
Perhaps his cartoon appearance is his most notable and ignomious position. After having been trapped deep in ice for millennia he was thawed out by the Decepticons. Although happy to see his old pal Starscream he didn't agree with the 'Con ideology (pamphlets available from S. Wave, By The Spacebridge, The Dull Grey District, Cybertron, ring top bell) and ended up supporting the Autobots and performing the penultimate sacrifice by becoming refrozen.
The grateful Autobots dug him out an indeterminate amount of time later when they needed a ride to the set of The Crystal Skull and continued to keep him around for further road trips and so on until they traded him in for Omega Supreme. He blended into the background so perfectly that the Decepticons couldn't even be bothered to kill him in the movie. Ouch.
It should be clear by the Jetfire figure's size, complexity and decent looking robot mode that he comes from a very different line to the various Diaclones, Micromans, Legos, Barbies etc. that made up most of the rest of Transformers. Controversially, he came from the popular anime Macross. Thanks to Takara's awkward relationship with Bandai and Harmony Gold's acquisition of the Macross animation rights, Jetfire existed in a legal grey area the size of Shockwave's phone bill. The consequence of this was that Jetfire was allowed to appear in the cartoon provided he didn't resemble himself in name, form or desirability. Instead he was lumbered with a silly name and an ugly Floro Dery design with a generic face.
"Ta...WHATTHEHELLISTHAT!?!"

The face of awkward compromise.
For obvious reasons neither Takara or Hasbro have ever been able to reissue the G1 Jetfire (either buy a Macross figure and some paint or save up your pocket money for a few centuries to buy the G1 figure) but there have been a couple of other versions of this character since then that aimed to combine the maximum of toy-accuracy with the minimum of getting-sued-by-Bandai-ary. The Titanium figure looks fantastic but I have no idea who it's supposed to be. The Henkei/Classics figure is pretty much the best rendition we could hope for with some nice detachable rocket-booster/cannons for greater G1-iness and the boring Dery-face with a removable toy-accurate helmet (unfortunately this makes his toy-accurate head look too big) and a ton of playability with a jet mode that tap dances all over the Lawn of Legal Actionability.

"Hello, Evil Sidekicks' Union? I have a complaint."
In the movie after committing genocide on the 1984-85 Autobot line the Decepticons were a little pooped. The remains of the battered Megatron, Thundercracker, Skywarp and the Insecticons were found by Orson Welles who gave them new bodies and a pointy new spaceship with only 600 lightyears on the clock. Thundercracker was transformed into the coolest of the Uncronians: Scourge.
Scourge was an early army-builder with a group of identical copies of himself with differing but negative personalities (years later, Animated Starscream would totally rip off this schtick) called the Sweeps. Though he played second-fiddle most of the time to Cyclonus he often proved to be more cunning and thoughtful, especially that time when he grabbed the Matrix and turned into Toxie. He also had excellent long range vision and the best moustasche in a movie full of quality 'tashes.
During The Rebirth Scourge, Cyclonus and a bunch of other 'Cons ended up on the planet Nebulos where some of the local became chatty guns and Scourge became a Targetmaster. Fracas was just as pompous as Scourge.

"The recoil keeps giving me whiplash."
Scourge was one of the earliest and best-known Shellformers. Essentially his wings and the back of his legs form the outside of his ship mode meaning that his body is flying around inside an unusually gaudy and mobile coffin. On the plus side that means his proportions and articulation are superior to other figures of the period.
"What I lack in accuracy, I make up for in lack of weapons and size!"

"Mecca's this way, right?"
Kitbashed from parts of Bombshell by Unicron, Cyclonus is pretty much the exact opposite of Starscream. Though they are both seconds-in-command, Cyclonus lacks the arrogance, cowardice and treachery of his forebear and is instead modest, honourable and endlessly loyal. He also has a duplicate known as his Armada but he got rid of it, presumable to avoid confusion with Cyclonus from Armada.
The figure of Cyclonus has a passable robot mode with big chunky fists and an Alt mode that's incredibly accurate provided you chop his legs off at the knee (if I were you, and I'm not saying I am, I wouldn't chop any G1 figures. Not even Starscream).
Much like Scourge he too got a Targetmaster partner.
"I just felt a great awesome in the force, as though millions of hands suddenly high-fived at once."
Given such a popular character and given that the name was still in use it was no great surprise that a G1 Cyclonus figure would see light again. What was a surprise was the quality. The figure came in two fruity flavours: Ridiculously Dark Universe Grape and Looks Too Lavender In Photos But Is Actually Good In Person Henkei...um...Apple. While the Alt mode of this figure suffered a little from being over-detailed and being a deluxe size, the robot mode was impossibly screen accurate and generally beautiful. As an added bonus, the figure came with the Targetmaster partner Nightstick who was well articulated and in gun mode could be mounted directly to Cyclonus' wrist for full-on Japanwank effect.
The Technobots (FOR SCIENCE) had two space...jet...things among their ranks. Strafe transformed into a craft made up partly of flying and mostly of shooting with two huge cannons and no visible cockpit. In robot mode he could duel-wield the cannons as pistols, guns akimbo (the second best kind of akimbo). Scattershot turned into an indistinct but cool ship with a big cannon in front. He looked pretty damn sweet. Unfortunately these guys aren't in the gallery.

"Why did I say "Roger-dodger"? There's no coming back from that."
Cloudraker represents the slightly-less-losery fifty percent of the Autobot clones. He still sucks though.

I find his youthful awesome most refreshing.
Brainstorm is everything a 90's Transformer should be: chunky, stylish, a little gimmicky and a lovely shade of sea green. I don't actually own this guy but he's the next grail on my list. If anyone wants to send me him for "review" purposes you're more than welcome to.
He thinks that Transformers would operate better with human partners. Just like 90% of fangirls.

Clockwise from front: Cyclonus, Scourge, Misfire, Triggerhappy, Slugslinger and Wal...GODDAMMIT WHERE IS HE?!?
When Scourge and Cyclonus went to Nebulos (Yes we're back to that, try to keep up at the back) they took their jets and their pets. The pets became the Decepticon Headmasters while the previously inconspicuous jets became Targetmasters too.
Slugslinger is a sneak, a coward and a blowhard who enjoys making threats then running away. He can usually be found hiding behind Triggerhappy. He's like Starscream with the ambition taken out and a measure of common sense put in.
His jet mode is fairly unspectacular save for the fact that he has twin nosecones with twin cockpits. I'm sure I'm wrong about this and that the various military historians, aeronautical engineers and know-it-all-ologists out there will correct me but THAT IS NOT HOW PILOTING WORKS. Unless he's some sort of Driver's Ed. jet with seperate controls for the instructor.
Triggerhappy is a shy, retiring sort with a healthy respect for armistice and gun control. I am of course kidding! Triggerhappy is the image you see when you look up "Gun Nut" in your Geoff Senior's Illustrated Dictionary (I'd buy that in a heart beat). If you put a hot fembot one side of him and Sixgun on the other side his neck tendons would melt from the constant twisting.
His Alt mode is a large pair of cannons which happen to have a jet stuck through the middle of them.
Misfire never really got to let his personality show, sadly. He's a rarity among Decepticons in that he's hardworking and enthusiastic. Unfortunately this is paired with the type of aiming skills usually seen in your local public toilet. This guy couldn't hit the broad side of...well, Broadside.
His Alt mode is probably the sexiest of the 'Con Targetmasters. Not only does he manage to make magenta cool but he has a cool set of vanes at the front that resemble the antlers of a stag beetle. Wonder if he made Shrapnel feel inadequate.
Sixshot the Decepticon Sixchanger had, appropriately, six modes. These were: statue of a robot, weird...beast...thing, bizarre gun, oddly square but otherwise cool truck thing, actually cool ground-to-air missile launcher thing and...an imposing but incredibly unaerodynamic jet. This guy really needs an update with some posability and alt modes that look more like anything.
Speaking of unaerodynamic, the Autobot Sixchanger, Quickswitch had a cool if unwieldy flying fortress mode. I already defended him back in Boats.

Powerglide, the Cybertron years?
Raindance is a silly-named Autobot who transforms from a cassette tape into a heroic shuttlecock.
If anyone cares about Pretenders a few of them turn into the kind of things you might find in the bin next to the Royal Airforce's drawing board. The Autobot Pretenders in this category include Cloudburst, Skyhigh and Lookupintheskyisitabird. Also Gunrunner transformed into an orange blob which his backpack could carry around in the form of a jet.

"When there's trouble you call DW!"
Separately the Decepticon Powermasters, Dreadwind and Darkwing transform into fairly realistic jets but together they can form an unbelievably powerful force for
evil emo. Despite being incredibly powerful these two were both grumpy guses who generally hated everything but each other.
The Decepticon Pretenders were generally as unfortunate as their Autobot counterparts. Their numbers included Bomb-burst, Bugly and Roadgrabber.
1989 introduced a whole bunch more Pretenders, many of whom were given more filled out personalities by Furman's writing. Doubleheader lived up to his name with a twin-nose jet mode like Slugslinger's but in his case it stemmed from an extreme form of multitasking and Zaphod Beedlebroxing where his two heads handled different tasks.
The Ultra Pretenders were a great example of the strengths of gimmicks, as well as Hasbro's dictionary lacking the word "overkill" (In the Geoff Senior's Illustrated Dictionary a picture of Overkill the dinosaur can be found here). They consisted of a more-or-less scout-sized figure who could transform who could go inside a Deluxe-sized shell which could
also transform, which could
then go inside a big vehicular shell. I am not kidding you. Both these figures are great but due to being serious victims of Gold Plastic Syndrome they are extremely hard to get a hold of nowadays.
See if you can guess which comic-based Decepticon leader I'm describing: he's incredibly powerful, able to demolish whole groups of Autobots with no problems; he has no problem with beating up his own troops; he flickers between cunning lucidity and incredible lunacy in a heartbeat; he likes making speeches; he can only be defeated by whichever Prime is popular at the time. If you said Megatron, you're right! If you said Galvatron, you're right! If you said Shockwave, you're right! If you...yeah, you get the idea. Neither Budiansky of Furman could write more than one type of villain and Thunderwing was the same way.
Thunderwing was actually a regular Decepticon but you wouldn't know that since he hardly ever left the safety of his Pretender shell. Inside his shell he was a tall, near-indestructible samurai-looking guy with a very big gun who could transform into a cybertronian bomber. Like many Pretenders the figure was fairly weak but that hasn't stopped him commanding prices way higher than, say, Vroom (who?). Don't bet your bottom dollar that this guy's not gonna get some sort of homage or update in the future.
In 1990 the Actionmasters and Micromasters ruled the franchise. The Actionmaster Starscream came with a little jet that he could straddle like a pony and Gutcruncher (who?) had another jet, the back seat of which was a tank. Hmm.
Meanwhile in Japanese continuity things had drastically changed. Leadership of the Autobots was no longer restricted to those with the surname Prime.
"Bui! Bui! Bikutori! Toransufoamaa bikutori!"
Victory was the start of a strange and distinctly Japanese era for Transformers. While Pretenders and Micromasters held full reign in the West, Japanese figures started to owe more to anime and mecha traditions with Alt modes that were not even faintly Earth-based but instead became blocky and brash-coloured spacecraft. Star Saber was a good example of this with his huge, tricolor spaceship mode. Victory Leo could form a booster for this mode or could literally be a separate flying brick.
In Zone, Dai Atlas ("I'm from the valley, see") could transform into a white, orange and blue thing, further cementing the Japanese-exclusive reputation for being awesome while looking like nothing in particular. Similarly Sonic Bomber transformed into a black something. Together with Road Fire they could form Big Powered, a vast...thing.
In the Return Of Convoy series (I'll not spoil what happens in this series) Sky Garry appeared, who looked like nothing so much as a Lego Space set (in the best possible way).
Both Japan and Europe got the mighty Overlord, a figure who can form simultaneously a jet and a tank. In robot mode, his top half is a lady and his bottom half is a dude. Just like my holiday in Thailand!
That's about it for G1. Beast Wars, especially the Transmetal era, included a lot of figures with sort-of jet modes, not least of all Waspinator and Terrorsaur but since these guys are a bit forced I won't be including them here.

Amoral and psychotic with the personality of a gameshow host. But then, I repeat myself.
Jetstorm was one of the most fun things about Beast Machines with his sleazy, easy evil and his dark sense of humour. His toys were pretty cool too even though, like most of the vehicons, his figure had been made apparently without anyone from Mainframe or Hasbro talking to one another. Shame he got changed into an emo turkey.
Beast Machines also included a guy called Spy Streak (who?). The awesomeness of his name was directly proportional to his number of appearances in the show and the number of people who've heard of him.

"It's like something from H.P. Lovecraft! Only gay."
RiD Megatron has already shown up several times on this list (I think his default robot mode, his bat/gargoyle mode and THE CLAAAAAAAAWW are the only modes that haven't warranted a mention) so I won't go on too much about him. Only that he's awesome.

"Tell my fangirls I'm not gay!"
Armada was the first full-on, balls-to-the-wall, pants-on-the-ground homagewank (there ya go guys!) series with number of G1 figures revitalized. One of the most notable was Starscream who had turned from his skinny, gamine G1 form to a beefy, butch heavily-armed formed: Starscream on steroids. This fangirl-favourite was a pretty amazing figure with lots of fun features.
"Thunderclash is on the move! Thunderclash is loose!"
Thunderclash the Minicon came with Skywarp who was a retool of Starscream with more in the way of fins and vents. Not much to say about this one.

"My name is Storm Jet. Stop giggling."
Poor Storm Jet. Not only is he rarely seen outside of his gestalt form as Superion Maximus (titter) but despite his bio describing him as a gun-nut he doesn't actually have any guns. Must be frustrating.

"My name is Wing Saber. What? Stop laughing!"
Wing Saber transforms into an impossibly silly-looking Stealth Bomber covered in weapons and is a homage to Star Saber and a glove-and-boot set for Optimus.

"Oh I'm a little bit country. And I'm a little bit rock and roll."
I don't own this figure but Slugslinger is an obvious homage to G1 Slugslinger. He's apparently a bugger to transform.

"First a gun, then a tank, then a tank on a plane...I like where this is going."
Energon Megatron is a homage to G1 Galvatron and has started flirting with being a jet (at least he isn't flirting with Mirage) with a tank on the back. I have no idea where Hasbro got the idea that jets work better with tanks mounted to them but I don't really see it.
Cybertron had less homagewank than Energon but was full of some pretty neat Cybertronian jets:
"Bugger me Sideways!"
In Cybertron Sideways returns as a side-switching creep of mysterious origins with a cool jack-o-lantern face, a faction-switching symbol and a cool jet mode. Between his limited-release in Japan, his super-rare redeco Noizemaze Mass-Production and the use of this mold as BotCon Scourge and the Sweeps this guy is not so easy to get your hands on.

"You kids today and your Millennium Items, and your card games, and your loud music and your hula hoops and your hopscotch and your dungarees and your lollipops and your Sony Playstations and your voice-activiated light switches and your leather pants and your artificial insemination and your Blu-Ray Discs and your pierced scrotums..."
Vector Prime is a fantastic figure armed with both a sword and a Minicon/Targetmaster partner with the ability to transform into a seriously antique-looking but awesome spaceship. In the continuitywank of the comics he's one of the original Thirteen Transformers who popped from Primus' surface like zits. In the cartoon he's just a mysterious old guy with mad timetravel skillz. I thoroughly prefer the latter.
"I'm a dragster. I'm crazy, I'm a jet. I don't know why I smash Autobots."
In Cybertron Megatron continued to play at being a jet with a bonus car mode. Like RiD Megatron but less awesome and with half the modes.

"Autobots: inferior. Soundwave: gangsta"
Much like Sideways the Cybertron version of Soundwave was a refugee of the sinister and destroyed Planet X. Unlike Sideways who was sleazy and charasmatic Soundwave was his usual low key self, though his monotone dialogue was now peppered with fantastically out-of-place slang. This guy really didn't get enough screentime. I don't own any of the versions of this figure but I know he does come with a neat Laserbeak who transforms into a bomb (that sounds like a one time only kind of deal to me).

"You look familiar..."
I'm going to tar both the Titanium seeker mold and the Cybertron sort-of seeker mold with the same brush here. Both these figures were heavily influenced by the Seekers from the tedious War Within section of the Dreamworks comics which were in turn influenced by the so-called "tetrajets" or pyramid-fighters from the cartoon pilot Arrival from Cybertron.
tl;dr: My Starscream, it has three corners; three corners has my Starscream. And had it not three corners, it would not be my Starscream. Cybertron Starscream was a distillation of G1 Starscream into his purest, greediest, arrogantest, treacherousest form, but with the cowardice taken out. This version of Starscream ended up taking on the whole Autobot army at once (taking massive damage) and getting what he was after, the Cyber Planet Lock which transformed him into a planet-sized version of himself. His ego was still even bigger than that though.
After the realism of the Movie, Animated was a return to Sci-fi form. I won't be including Starscream, Lugnut and Blitzwing since their Alt modes are very heavily based on Earth vehicles, but a different matter is:

"Chekov, Pavel. Rank...Admiral."
Jetfire and Jetstorm were two of the later additions to the Animated series. Their unusual powers and abilities (control of fire and wind respectively, the ability to combine into a super robot or a super jet) came as a result of being infused with Decepticon technology. Whether or not this also caused their Russian accents is a question for the TF wiki (WILLIS!!)
Like much of Transformers their figures were much maligned before release and only minorly maligned (smooooth) after release. Their jet modes don't look like much and their combined robot mode looks a bit Two-facey but they have a great gimmick and lots of posability.
And that's it for entry number 2. If I've forgotten anything then...well, it's too late. There ain't no stoppin' this train we're on! (yes, entry number one is going to be Trains).
Galvatron: I tell you for the last time, Diem. Surrender, or there won't be an Earth left to protect! Surrender! This is your last chance, or—
Diem: You're bluffing, Galvatron.
Galvatron: I tell you this is the end!
Diem: You're right. It is, because I used the real 'ultimate weapon'...my list of the 26 Greatest Alt modes!