I chose mine because it was a good design and it was for low points. I hardly ever see anyone who has the same as mine. I think it also kind of shows I'm not some nerd who stays up on AG all night spamming game comments and rating games that I haven't played for points.
I often switch mine between some I like, but I always go back to the armatars I've used the longest and like the most, such as the bear, frog, unicorn, and blue shield.
I think this thread has lived long enough... Anyway, for some proper discussion on the subject (and considering people will still tell why they have or like a certain armatar in that thread) please refer to the General Armatar Discussion that is buried somewhere in this section some pages back.