The "Complete" books are great from a player point of view. You could probably also use them to add some interesting classes and PrCs to your NPCs.
Additional monster books, like Monster Manual II and III (I liked III a lot, haven't looked at IV in enough detail to judge) are good for variety. The Creature Collection by Sword and Sorcery is good, too (I liked the first one more than the next two).
Definitely the DMG II and the PHB II add a lot of new options to your game.
If you want to point your campaign in a more epic good versus evil direction you can try the Book of Exalted Deeds and Book of Vile Darkness (though these do contain a lot of mature themes and won't be good for a young or not-so-serious game).
My DM loves Sandstorm, though I've just flipped through it in the store. If your campaign has a lot of a certain environment (Sandstorm for deserts, Frostburn for cold, Stormwrack for ocean) then you might want to look at the appropriate book, though like I said, I've only flipped through these.