I've barely given the other ones the ol' college try, but from what I've read and learned at SVA (from pro inkers if that adds any weight to it for you) was that the #4 seemed to be the favorite. Not too thin, not too thick. I believe my old teacher Klaus Janson used a Rafael #4. Oh and make sure it's watercolor Round! No liners or any other kinds of tips.
i've only ever used a number two. at one time i used it for EVERYTHING. got tired of the set up and clean up. every once in a while i break it out to fuck around or tackle a line i think it's best suited for. otherwise it collects dust in the corner of my desk. watching what you do with one makes me jealous. makes me think i should break it back out.
I can get pretty weary of the whole cleaning process myself but it's all part of the ritual. Clean brush means a clean start. And the damned thing pays for itself. You just don't get lines like it with microns, or pitt pens, or pentel brush pens. It's downright sexy.