My personal favorites include the Spell Compendium, Magic Item Compendium and Player's Handbook II.
All of these are include stuff for all classes, all playstyles (that allow beyond core at least) and both new and experienced players. There will definately be something in each of these books that you can use.
Beyond these it depends on what you find interesting and your groups play style. The Fiedish Codexes get a lot of love around here and so you may want to look at those if you like reading up on D&D's demons and devils. I like Complete Mage but it won't do you much good if you don't like playing arcane casters. Book of Nine Swords has a lot of goodies for the fighters in the group but the style of the abilities at higher levels may not mesh with your groups prefered play style. Complete Scoundral has some fun stuff in it for the skill type classes but I don't like it quite as much as Complete Mage or Book of Nine Swords.
