I've seen the "favourite userpic" in three journals so far, slightly different each time :)
Here's how I'll word it:
Comment on this entry, and I will tell you which one of your icons is my favourite. (I may choose more than one if I can't decide which one I like best.) You can comment with the one of mine you like the most, or just with anything.
(Which is a composite of the three I saw.)
As for me, I like to use different userpics in different situations, so it's difficult to pick one favourite of my own. However, I kind of like the 'rgb' icon I'm using on this entry. I suppose it's a geeky kind of thing; at any rate, I can't really put into words why I like it except that the concept seems nifty to me.