The Energon Pub

I'd say either one of the Unicron Trilogy lines, Animated or Prime.
The figures are reasonably simple, especially compared to Movie figures, and all from finished lines. The Animated and Prime series were quite good, at least in my opinion.

I'd second Animated. It's got plenty of G1-reference goodness (e.g. Autobots are land vehicles, Decepticons fly, Bumblebee has hood-feet, etc.), but it also establishes its own universe (e.g. "Prime" is not the highest Autobot rank, the Great War is actually over, Sentinel Prime is still alive, etc.). The toys are solid figures, and many of them were cleverly designed to match the aesthetic of the show. There is a good chunk of convention-exclusive figures, but most of them are characters from ancillary media set after the conclusion of the show, so treat them as you will.

Glarryg

Yeah, Animated is on the top of my list as well. The figures are fun and colorful and ACCURATE and the line is healthy and full. Meaning, there are plenty on both sides and 90% of them had a character on screen. As for exclusives, as far as I remember, the only ones TFA had were the ones released at BotCon 2011, the Stunticons along with Sideswipe, Toxitron and Fisitron. And yes, plenty of G1 references. My 2nd choice would be Armada. The line had some figures that were just fillers for the line, but they were all fun toys to play with. And I loved both the comic and the cartoon.

I have two in mind. One is based on personal experience, the other on anecdotal observation.

First suggestion - Cybertron. Easily the best of the Unicron Trilogy lines when viewing things through an objective lens. Unobtrusive gimmicks for the most part (this comes up next podcast in more detail, for the record), high quality plastic/builds, and the transformations are satisfying without being frustrating. There are probably just enough G1 references along with the mostly new cast of characters to make you feel at home. Of lines that I can truly look at and say "this is complete", with no new releases coming out, I think the toys are the most like the G1 toys in terms of the engineering, though they're probably more similar to '86-'89 G1 than '84 and '85.

The show is ok. I personally love it, but would admit that it'd be a hard one to sit through at full attention again. The Speed Planet arc is brutal, and drags a lot of the show down. The last 1/3 or so is super good for what it is, which is a Saturday morning "eat your cereal and scope out what toys you want your parents to buy at the store this weekend" half-hour romp. There's a continuous storyline throughout, but it's pretty tough to get lost and not know what's going on even just picking something at random.

So the other one I'd say has potential for you would be the 2001 Robots in Disguise line. I should preface this by saying that I've seen very little of the show since it hasn't had an official DVD release here in the states, so I can't speak much for that part. I bring this one up because I've observed that a lot of my friends that are really big into earlier G1 are also really into this series. X-Brawn, Side Burn, and Prowl take a short while to get used to (use the instructions the first time out!), but they're not horribly tough and they give me a bit of that '84 car bot feeling. Optimus and Ultra Magnus are really great, in my opinion, though Ultra Prime can be a bit frustrating to assemble. At least that's optional!

The rest of the series' original mold toys are pretty cool as well, featuring one of the best Megatron toy designs probably ever as well as a neat combiner team that takes "Scramble City" to a funky, but still pretty fun, new arena. Oh, and the train bots, they are good. Real good.

Outside of that, there are molds from every other Transformers series that existed up to that point in the line as well, many of whom are not just oddball filler - they actually appear in the show! It's a line that distills a lot of the best of the first 17 years of Transformers into one neat package, so if you've been isolating yourself to G1 it can also be a super fun way to experience some of the other stuff you may have not paid attention to without going into too deep of a rabbit hole.

As for rarities, yes, they truly do exist in each line, but they're also truly isolated to Takara releases. If you just were to go with Hasbro product, a good bit of this stuff is not expensive at all. Some of the "Leader" type figures (Cybertron Megatron, OP, and Primus or RiD OP and Magnus) have inflated a good bit from their original retail pricing but not so much that they're extremely burdensome.

The only "that was in the cartoon, why is there no Hasbro toy!?" moments for each are limited. For RiD, it's Brave Maximus. As the Urban Legend goes, it couldn't pass a "Drop Test" so Hasbro couldn't release it. I don't know precisely what that is because I'm not a Toy Safety industry expert and won't pretend to be one. Anyhow, in Cybertron this is isolated to redecos: no Blue Rumble or Coby Rumble, and no "Mass Production" color Sideways, but I think most that watch the show probably aren't going to pine for those too hard. If you do, everything I just mentioned outside of the Blue Rumble is legit expensive and I'm sorry.

If you just want a taste of each line, without spending a lot, I'd say get one of the "Car Brothers" from RiD (X-Brawn, Side Burn, and Prowl). Try to get one of the first couple decos of them actually released under the RiD banner. They got repainted a ton in Universe 1.0/etc and some of the later uses of the molds are fragile as can be. For Cybertron, Voyager Vector Prime, Deluxe Hot Shot or Landmine, and Scout Scattorshot are good examples of what to expect from that line. I don't think any of them are super pricey, though I haven't looked in a long long time either.

Hope that helps!

Part of the problem is that many of the lines that are complete at this point will be expensive to track down just because you're going to have to buy old (rare) toys. You'll be Ebaying a lot of these.

Prime. Especially if you limit yourself to the on-screen characters. Makes for a good small display.

Though the aesthetics are almost bay-verse like.

If you're coming from mainly G1, I'd say RiD(01) would be the way to go, due to the way that line features a nice mix of new, redecoed g1/g2, and even beast stuff.

I'm not sure if Car Robots/RID would be your cup of tea, considering the car brothers and OP, Magnus are are about half as complicated as Alternators.

Unicron Trilogy(Armada/Energon/Cybertron) would be a better bet with simplified designs and being made around TF's 20th anniversary(per TT), chock-full of G1 references. The show has its hits and misses, but I only had the chance to watch the Japanese versions of the series.

I also think Animated would be a good way to go. They took G1 ideas and kind of went a different, unique direction. They toys are fun to play with and they look great on display. It's like they took the G1 characterizations and crafted the figures to fit their personalities. The figures look like they jumped from the show into your hand.

They show was it's own story and universe but was filled with obvious (and not-so-obvious) references. Great stuff.

I'll second Animated and Cybertron. Animated had gorgeous engineering and some really cool looking toys (though as with all lines there are some stinkers). The show is great too, or so I'm told. Cybertron is in my opinion the golden age of Transformers toylines- nearly every toy was a gem. The show's pretty good in both languages too. Robots In Disguise is a good line. The show (AFAIK) had its good bits and its bad bits but is generally a lot of fun regardless of what language you watch it in. The toys are sublime.

One toyline that might be worth collecting is Energon. The show is utterly rancid, but the toys are great. A lot of the early Autobots were somewhat hindered by the combining gimmick but there are some gems (Landmine and Bulkhead stand out, mainly because they don't combine) and the Decepticons are universally brilliant.

As far as Energon goes, here's some toys I'd recommend (note that I don't own all of them but I know a lot about most of them, through intense research):
-Any and all of the scouts, but especially Cruellock, Divebomb, the tank molds from the Bruticus set, Strongarm and the "aerialbot" limbs
-Mirage/Dreadwing
-Megatron/Galvatron (either size)
-Landmine/Landquake
-Scorponok (and any redecoes thereof)
-Snow Cat
-Bulkhead
-Sharkticon
-Slugslinger (IF you can stomach the fiddly transformation)
-Tow-line