Some tough choices!
For "Splat" Books:
1) Draconimicon- 'Cuz dragons RAWK!
2) Lords of Madness- Some of my favorite monsters gathered together in one book? COOL!
3) COmplete Champion- A concept I like, backed with some solid ideas and excellent art.
As an addendum?
The two books I had VERY high hopes for that were subsequently dashed? "Libris Mortis." It is, as they say, "teh suck." And "Fiendish Codex II."
For "Adventures/Campaign Settings"?
1) Midnight
2) Age of Worms
3) Savage Tide