Kickback's. Because none of the rest of us can say Derrick Wyatt (or anyone affiliated with official Transformers production, for that matter) took the time to specially make a personal avatar.
I vote for Vangelus. Especialy the most recent one, which combines his Animated incarnatrion and the infamous 'scared baby'. I need to make a new avatar for myself now that I think about it, my current one only relates to my WIP Energon Nemesis Prime, which has been stalled as of late.
I would have to say Vangelus's avatar just because it has a cartoon version of him and a random baby of non known portions... why not?
I don't know about best, but I like to think mine's one of the creepiest, possibly tied with Spiderus' avatar.
<----looks at thread. Sighs because of all the ******ed comments, leaves, and goes have a conversation with Denny Crane about declining IQs or some shit like that.