Recent favorites;
Was Heroes of Horror this year? I forget, but I bought it this year, so that counts for me.

The Dread Necromancer totally works for me, plus, as an Al-Qadim goob, it was nice to at least see the name Kazarabet again, even if the axe-weilding asian hottie was very much not the Kazarabet I remember...
Player's Handbook 2. Like Unearthed Arcana before it, it just expanded the possibilities in all directions. Very cool. The Duskblade, IMO, is what the Hexblade so miserably failed to be.
Complete Mage. Reserve Feats are a cool idea, and WoTC made them even cooler than WW did back in Vigil Watch: Asaatthi as 'Locus Feats.'
Tome of Magic. On the fence, kinda, I liked all three ideas, but the implementation of the Shadowcaster and Truenamer didn't work so well for me mechanically. (Although Ari's suggested tweaks resolve any qualms I have about the Shadowcaster.)
Not so much a favorite;
World's Largest Dungeon. Ugh. How much do I loathe adventures that have lists of PCs that the material can't handle. If it's big, it should be inclusive, yanno, instead of being 'World's Fattest Book you may not be able to use.'
Cityscape. I wanted to like this one, but it didn't blow up my skirt. It deserves points for veering away from the occasionally scattershot kitchen-sink format of Stormwrack / Frostburn / Sandstorm, but I think it veered too far and didn't have enough 'crunch' for my tastes.
Tome of Battle. I don't own this book, and like Faiths of Eberron, it's on that list of books I call, 'If the guys over on the Character Optimization forums cackle maniacally while stroking their mustachios about about how gloriously broken it is, I'm not even gonna buy it, for the sake of my sanity...' For some sort of Exalted / Wuxia-style game where the Fighter-types are supposed to have all sorts of cinematic 'kill everybody in 60' with a toothpick' powers, fine, but I don't plan on playing such a game. I'm too old to 'get' anime.