From the list, I'd have to say this one:
I had it for ages when I first joined OF; I only stopped using it when somebody else started usng it as well. On message boards, I always like to have an avatar that no-one else has.
Right now, I have this one:
[link]
(I'm posting it here because I might have changed it by the time people read this topic). I nicked it from a screenshot of JArKaos. EDIT: You have to copy/paste its url into the address bar to see it. I tried using some code that would let you see it just by clicking the link, but I keep getting it wrong and I can't be bothered to wrestle with it anymore.