Cybertron's the most diverse line since Generation 1 when it comes to alternate modes and gimmicks activated. It's no wonder it's my favourite line next to Beast Wars, Classics and RiD. Here are my picks:
ScoutsRansackOne of the very few motorcycles to pull off a great robot mode. You can either get the red original, or the GTS version which kinda looks like Animated Prowl with additional white and translucent green.
WrecklooseA Hasbro-only mold, he's perfect if you're looking for a Beast Wars feel, especially the Transmetal 2's. His gimmick is a bit less impressive, it makes him look like he's auditioning for Santa's spare reindeer.
Hardtop / SwindleAnother Hasbro-only mold. He's simple, design is nice and clean, nothing's overdone or underdone. If you're into G1, get Swindle, even if his colors are based on the Alternator version instead of the G1 original.
ScrapmetalThe perfect army builder, as in the show Scrapmetal are like locusts, traveling in swarms. Transformation is pretty straight forward, and he comes in 2 to 3 flavors: red, yellow and blue (a VERY common assortment of colors in any line). You'll have trouble tracking down the blue though, as that color's Japan-only. I know I have trouble...
ShortroundAgain, a Hasbro-only mold, and unique in the fact he's a hovercraft. You may be turned off by his large arms though.
DeluxeHot Shot / ExcellionEasy peasy to transform, and just looks sleek as a car. You can him in blue or red, the red one completes the homage to G1 Hot Rod. Little known fact: his transformation almost matches that of Volfogg from
King of Braves GaoGaiGarSidewaysThe other side of the spectrum, for Sideways you really need the instructions. His sharp angels give him a very evil look, and his faction switching gimmick is perfect for his role as all-round trouble maker.
DownshiftA classic car makes a classic robot. Blocky, yet modern.
VoyagerVector PrimeChuckfull of detail, you really need to get this one. His head even looks like the Autobot symbol, you can tell he has to look ancient.
Cybertron Defense ScattorshotWho doesn't like a missile carrier tank transforming into a well-armed robot?
UltraMenasorAs the name implies, he looks menacing, with a claw and drill for hands. And in the show, he's REALLY big as a Giant Planet inhabitant.
Leader ClassOptimus PrimeNo explanation needed.
MetroplexThere! I said it, I'm recommending Metroplex!

Sure, his vehicle mode doesn't look much like anything other than a robot performing yoga, but that's part of the charm. Tip: Look up "Megalo Convoy transformation" on YouTube, and you'll see how majestic he looks and transforms. Grab a few Legends for a good sense of scale.