I used to use Lt. Surge as my avatar on Pokémon Showdown, actually. I like Lt. Surge, because he's a hardened military veteran who basically breaks down into a squeeing fangirl when he sees you have a Pikachu in HGSS, to the point where seeing you with a Pikachu makes him immediately offer you his phone number and offer to trade you his own Pikachu, which he adorably named Volty (it's French in English versions, but everywhere else it's English, which makes it pretty clear it's intended to be from America like he is). Plus, he looks like Guile. That's a plus.
I'm also a fan of Blaine. He's a highly-energetic old scientist who lives on an active volcano. He also makes his gym into a trivia quiz, because that's just what he likes, I guess. He also looks a bit like Dr. Eggman, which is a plus.
Dawn stuck with me for some reason, probably because I haven't watched the Anime since the DP arc I guess. ((Piplup became my favorite 'mon coincidently.))
I personally prefer a lot of the Gen 4 Trainers like Byron and Aaron but not Cynthia (who is completely overrated and is everywhere). But my true favorite has to be Miror B. I mean you gotta be cool if you have two different themes.