I like anything that is "cognitively dissonant". Any avatar that says one thing when the context explains the opposite, at least when it's not obvious, when it's subtle and doesn't rely on cliches.
On that behalf, Night's multiple avatars are often very interesting, and difficult to miss. "Save Darfur", for instance.
Into It is also a good choice.
On a different mood, Edgar's are not that bad, either. Especially because each time I notice them, they look like the promise of a silly laughter when I discover the message next to it.
Somehow, you can say Edgar's has "trained" me into liking his avatars, just like Pavlov's dog. There's always a reward when I spot one of them.
Well... If I want to stay popular here, I'd better not give that list.
I do not like Avatars that are designed to impress people, that pretend to exhude confidence because most of the time, they don't.
Any Avatar that portrays a superhero, for instance.
"A man who only drinks water has a secret to hide from his fellow-men" -Baudelaire
Another thing: I don't like avatars that make me think negatively of the person, or make me read negative connotation into the person's posts, even when they don't mean it that way. I also don't like avatars that scream "Hey! Look at me! Look how f*cking COOL I am!" Interpret that how you will... I'm not going to name anyone in particular, because I don't want to be anyone's enemy.
I like the avatars that make me think positively of the person, because they seem fun/nice. Those people include Domino, d@v3, and others I can't remember at the moment...