the DMG and the Manual of the Planes. Especially the Manual of the Planes... I only wished that they had done more with old planescape material especially Sigil
Seeing the numbers I realize that I'm the only person so far that didn't marked the three core books. I already gave them to a friend who's running a game to open space to the revised edition.
I think this proves this is a forum for DMs... Every one who voted (save Ron) has all of the three corebooks.
As a favorite... Well, I don't really know. I like the MotP, OA, and the Psionics Handbook. I have voted I had the BoVD, but actually, I don't have it right now -- I should get it as soon as my slugglish FLGS finally receive one...
We have one player too! Someone marked PHB and not the DMG or MM! Woohoo!
There's an oldschool rave track with a sample going "Everybody wants to be a DJ" that could be easily adapted for ENWorld purposes...."Everybody wants to be a DM"....
I actually own them all, but chose to respond based on what I would have purchased if I hadn't gotten them for free. The ones I think I would be without include Deities & Demigods, Epic Level Handbook, Faiths & Pantheons, Races of Faerun, Arms & Equipment Guide, Hero Builder's Guidebook, and the two Gazeteers.
Well, all the players I've had in my campaigns, for the past couple of years, have owned all three corebooks, and few of those players ever serve as a DM.
Frankly, I wish players who have no aspirations or intentions of being a DM would own just the Player's Handbook, of the three core rulebooks. But, then, that would not bode well for WotC's business.
Oh, and of all the books and supplements listed, my favorite is, by far, the Player's Handbook. Of all those listed, it is a veritable masterpiece. (Of course, it does have its flaws... )