If I had a kanji avatar, I'd probably have 龍 (although that one's been taken by someone else and I think making duplicate avatars is potentially confusing). I don't know why, that's always been my favorite kanji since I discovered it. I guess it's 'cause Chinese dragons are cool, and this kanji has more... I dunno... dignity to it than the simplified version (竜), but I don't really care about Chinese dragons... I just like the kanji for some reason.
鵬 is one of two kanji on one of my official inkans here in Japan. I used to use it for my bank account and whatnot, but now its just sitting around collecting dust because I've since changed places of employment and they requested I make a new inkan.
Another reason is I often forget how to write this kanji (I'm not yet to it in RtK) and I like the meaning.
I didn't use one at all until I came across a heisig keyword with paddy in it... I thought, well, at least no one else will use something as lame as paddy-ridge....
no offense to anyone who is actually using an avatar as lame as paddy-ridge