My personal picks (no offense to anyone, I'm just being honest here), based on originality, the picture quality, and the harmony between username, avatar, and what I can determine the personality of the particular SYMian to be:
Best - Unambiguously and unapologetically mine! Ha ha!
Worst - fable's. To this day, I have no idea why he chose a chess piece for an avatar. Besides, a knight is practically useless without a bishop and a pawn!
Most Appropriate - Mr. Sleep's if indeed that dwarf is the correct one ("Sleepy").