Overanalyzed, much the way Baseball Hall of Fame voters handle their ballots:
1). Hot Rod/Rodimus Prime: The second most prominent Autobot leader, he starred in the 1986 movie, anchored the entire third season of the G1 cartoon, and honestly has had more character development than Optimus Prime ever got. He was only a minor player in the '80s comics (at least in the US), but he's now getting a big chance to shine in the IDW-verse. He's had a boatload of toys, ranging from good to truly excellent. He's also the only non-repaint Masterpiece toy that's not already in. How is this candidacy not a slam-dunk?
2). Unicron: The most feared force in Transformers history and a prominent part of their origin in many mythologies. He's played a role in virtually every Transformers continuity. He was the driving force behind the 1986 movie. The only possible arguments against him: He's not technically a Transformer, insofar as he is not a Cybertronian, and his toy resume isn't stellar.
3). Beast Wars Megatron: If we're going to have a Beast Wars character on the ballot, this is my vote. He had more character than most Decepticon leaders, and I loved the way he always seemed to be two steps ahead of the Autobots. Plus, he had three awesome toys.
4). Galvatron: I know, I know, this ballot is 1986 heavy. But seriously, Galvatron was awesome in the movie, truly interesting in Season 3 of the cartoon (seriously, watch Webworld if you don't remember), and absolutely *killer* in the UK comics, and to a lesser extent the US comics (Transformers #67 is an all-time favorite of mine.)
5). Jhiaxus: My fanboy pick. Can't justify it on a career achievements basis, obviously. But maybe if we all coalesce around him Hasbro will finally give us a toy of him.