PHB2 I love for the high-level feats, not so much for the new base classes and variants (some of these, like the Shapeshift Druid variant, I really dislike, and are not well thought out).
MIC is just fabulous. Great items, plus a list at the end of all the items in both this book and the DMG with a convenient summary of their powers.
The Complete books are even better than the above books, depending on your love for the classes they offer expanded options for. My personal favorite is the Complete Adventurer, which I consider indispensable for the inclusion of the Scout, my favorite base class. If you like exploratory campaigns, you have to check out the Scout.
If you like spellcasters the Spell Compendium is great, but the Complete books for the spellcasting classes are almost essential as well.
I'm not super fond of the Races books, but have found Races of the Wild to be the most useful.
