"true" favorite: The Quintessons
Decepticon: Rumble. little guy with a huge attitude. and the weird earthquake forming jackhammers he had, powerful enough to put down someone like Devastator.
Autobot: Autobot X.
http://transformers.wikia.com/wiki/Autobot_X
I am from back in the day when there was only transformers. My favorite autobot would have to be either Bumble Bee, or Jazz. My favorite d con would have to be soundwave, because he had that bird that could run reconisense, and then come back, and transform in to a VHS cassette, and soundwaves chest cavity doubled as a VHS player.
Autobot:
Ironhide... he was so kickass and knew when to be serious and when to have fun. I also loved his sarcasm =D He fit in well to the leadership role when Optimus was not around and was never afraid to go for it! Of course Bee and Arcee come in close behind him!
Decepticon:
Shockwave. I always liked Starscream, Soundwave and Barricade but Shockwave was great! I always saw him as more powerful than Megatron but he had such little desire to actually lead that he never tried to take over. I mean, even in the live action DOTM, even Epps asked why the Decepticons always had the good sh*t because he took a building down with such ease.