What is wrong with you people!? How can anyone not put down Gary Gygax as one of their three!? This man is a legend, the creator of D&D; I'd still pick him first even if he were comatose and in an iron lung.
Beyond that, Joe Browning is high on my list and then the gigantic tie game for third commences. So, like Teflon Billy (who I can vouch from experience is one of the finest GMs in Christendom), I'm only going to pick a top two -- there are too many people I've met on here that I'd be interested in gaming with to make a legitimate second choice.
EDIT: Actually, screw the waffling, I'm picking alsih20 as my third. (I don't have to pick Teflon Billy 'cause I get to be in his post-apocalyptic game anyway.)