Like many, for me it started with G1. However, Beast Wars is still the best televisionseries IMO. Solid story writing, well developed characters and the tie-in with the previous G1 series while still standing on it's own, are just a few of the reasons. But I will admit that Prime is a serious second contender. The gap is seriously narrowing. If Prime goes on like it does now and they not only develop the current characters as is but also dare to expand the character list, then it will give BW some serious competition.
Tough call. I guess mine would have to be the combined show that is Beast Wars/Beast Machines. Masterforce is a close runner-up, but the combination of its '80s and Japanese tropes really hold it back. G1 is always fun, and if all of Prime were as good as its first five episodes, it may actually have topped my list.
Animated, no Beast Wars, no Rescue Bots! Eh, I'll go with the newest one, Rescue Bots... Which isn't on the list.