Movie Scorponok, for being a wicked little critter. Movie Brawl--just looks great. Universe 2.0 Warpath--knees AND elbows? Seriously? With hips and shoulders thrown in for good measure! Universe 2.0 Wheelie--finally, a real Wheelie toy, and he carries a concealed weapon. What a badass Universe 2.0 Cosmos--he's great, and even greater with the "shoulder" mod. Animated Prime, Bumblebee, Prowl--They pull off nearly the same transformations as their larger counterparts and look great doing it. Lots of great choices in recent lines.
I would have to go with Warpath. Unlike most other legends, he seems like an actual update to his old figure rather then a scaled back economy version of someone larger. Beachcomber and Brawl are that way to, but they seemed to do the best job on Warpath. I was dissapointed with Universe Bumblebee- odd proportions and limited articulation even for a legends. I was even more dissapointed with Cosmos, even though everyone seems to like him, he's really hard to pose since he's poorly balanced, his arms are huge and of course oddly shaped, and he has a giant stinger that stands out like.... like a giant stinger. Being a character that never gets updates, I wanted a lot more, but even compared to other legends he doesn't stack up.
I love ROTF legends Jetfire. He's fairly accurate, has pretty good articulation, the alt mode is awesome, and he combines with Prime!
If I had to limit it to one per series: Cybertron Evac TFTM Ironhide Universe Wheelie Animated Bumblebee ROTF Optimus(thus far). There's lots of other great ones too.
2007 Movie Megatron, for he is the lord and emperor of Great Britain, particularly the now-lost Woolworths prefecture.
Since Legends Starscream mould goes without saying, I'm going to chime in and say ROTF Jetfire is the most fun I've had with a toy that small.
