While I agree that AU, Draconomicon and UA all are fantastic and deserve awards, I am going to go the slightly less traveled route, and say WotC's Underdark. I just finished reading it, and I think it has the perfect balance of crunch and fluff. And oh, the fluff. I don't run a game in the Realms, but I am soooo going to use so very many of the ideas, places and people they present in this book in my own game. It is an excellant book.
Jason