While I like most of the mane six ponies, I usually choose Twilight, AJ or Rarity as my avatar. Perhaps because I like purple, orange and white as colors, although it could also be because of other reasons that I am not aware of.
I prefer something simple, yet elegant. It must harmonize with the design of the forum but also be pleasing as a picture on its own. So it needs to be well drawn and show-accurate. To spice things up a bit, perhaps because I dislike most things that are maintsream, I like having a small twist in my avatars. In this case (at least for my current avatar), that would be the suggestive look from Rarity. Not that I'm implying anything. :I
In conclusion, I would assume the following about myself:
-I like being organized (simple yet asthetically pleasing avatar)
-I put an emphasis on matching/harmonizing color schemes (also evident from the colors I choose in my sig)
-It's hard to for me to choose favorites (I indicated AJ as my fav in my profile, yet I have a Raity avatar)
-I say I don't like following the crowd, yet I do most of the time (by choosing a pony avatar), which to some extent may indicate insecurity or hypocrisy
-Also I may be a clopper :/