Tough call, but I think I'll go with Thuldrin Kreed from our Unlikely Heroes campaign. We was both our nemesis and our chief employer, and probably the most fleshed-out NPC I've met in a game so far. The DM said the character reminded him of Gene Hackman's character in - well, some Western. (Sorry, Western fans, I just don't find the genre very entertaining or memorable. Westerns are too manly to hold my interest.) Also, Kreed had a bodyguard named, um... Payden? Paydum? Paycheck? Payday? Never did get that right… Anyway, Kreed’s henchman was a pretty cool character too.