Megatron, No question. Galvatron was an idiot and he had no real foil. I would've voted for Star Scream but he wasn't an option.
Galvatron was cool in the movie, but extremely lame in the cartoon series. His voice sounded like fred from scooby doo. In my opinion, the story line and animation went down for the series after the movie. In many ways, it almost looked like it was animated by hannabarbara.
Does star scream count as a leader? I would have put him as an option, but I don't know if he counts.