I really like how you need to have a certain amount of armor points in order to use most armatars. I also like how their are seasonal armatars to pick from, in case you get board of the regular ones.
As with tegan190, I have had my armatar since I started and I don't think I will ever change.
I think most people want to stand out from others and have their own identity. That's why there really isn't a popular armatar. Seasonals contradict that a bit, but that is because they only last a limited time.
In the forums is seems like nobody "opular" changes their armartars. However, if you check the comment section in a game, it is loaded with a bunch of 0AP armartars.
I used the blue butterfly since I unlocked it and didn't change it until the seasonals came. Then I decided to switch to the armatars I created. I plan to keep the one I'm currently using a little longer, unless I manage to draw a masterpiece and I force myself to switch it again.
I've been keeping seasonal armatars long past their season. I'll probably keep this one for a while, or at least a Christmas one. The seasonals are my favorite update to AG that I've seen.