1. Spell Compendium - One source for spells... and 25 less pounds to carry around.
2. Players Handbook 2 - Wonderful new core classes
3. Complete Arcane - I have used more feats from this one book (with the exception of the PHB)
4. Fiendish Codex I - Great story ideas in this book
5. Libris Mortis - Undead love, and lots of it.
6. Heros of Horror - I am a sucker for scary things
7. Magic Item Compendium (and moving upwards on my list) - Magic item tables in the back make it worth the price of admission.
8. Tome of Battle: Book of Nine Swords - Nifty mechanics
9. Complete Adventurer
10. Stormwrack - I happen to find this very useful at the moment, but then I am running a seafaring campaign