I've gotta go with Cyclonus. He was the most intelligent and sinister of the Decepticons, a brilliant and fearless warrior. He repeatedly kicked the crap out of that chump, Ultra Magnus. His ship form was the coolest ship form in anything, ever. Perhaps his only downfall was that he had so much integrity that he always remained loyal to that maniac, Galvatron. We all know he should've beaten the shit out of Galvatron and led the Decepticons himself; they would've defeated the Autobots without question. Most of the Transformers merchandise that I had as a little kid is now just a distant memory, but I still own a Cyclonus figure.
If I had to choose a favorite Autobot, I'd pick Red Alert. I loved that episode where he went crazy and teamed up with Starscream before beating him down at the end. It's too bad he didn't get more screen time.
As far as comics go (I read the first few way back in the day), Shockwave was a complete badass, tossing Megatron around like a ragdoll and capturing the severed head of Optimus Prime. Man, those early Transformers comics ruled.