I have two, one single and one a group of bumpers. My favorite single bumper is Astrotrain's. It shows off his triple-changing ability, and it's very visually dynamic. The Headmasters' bumpers are my favorite group of bumpers. Action packed, and they have more interesting scenery than the American G1 bumpers. I especially like Weirdwolf's. What's yours?
I like a lot of the bumpers, so I will stick with my favorites from each season. For season 1, my favorite bumper would be Lazerbeak. For season 2, it would either be Omega Supreme or Astrotrain. For season 3, it would be Ultra Magnus.