In no particular order, here are the D&D books that I really dig on.
The new classes are intriguing, the feats are fun, but it's the alternate class abilities section that really makes the Player's Handbook II sing. The ranger, rogue, and monk alternates have made regular appearances. And the new duskblade base class is probably the best warriormage class I've ever seen.
Do you remember all those tiny pamphlet adventures from the early days of 3E? Adventure I from AEG compiles the best of them, fleshes 'em out, adds some editorial polish, and upgrades the stats to 3.5 standards. There's a whole campaign worth of one evening adventures in this book.
Tome of Battle: The Book of Nine Swords might just be my current flavor-of-the-month, but I really enjoyed trying out some martial maneuvers last night. And Doug's character in my Eberron game has made great use of the Crusader class. If you like playing the melee monster but find standard fighters to be a little uninspiring, this is the book for you. With this book in play you can unleash your inner hackmonkey but get all the mechanical toys of a wizard.
I haven't put Arcana Evolved to use yet, but I simply adore the new classes and most of the new races. The magic system is an excellent tweaking of core D&D magic, while not going so far afield as to make it unrecognizable. The setting sometimes leaves me cold, but it certainly does a good job of tying all the new mechanical toys together.
The new Unearthed Arcana is the book that got me to make the jump to 3.5. Many of the best mechanical alternatives from the wilds of the OGL have been gathered in this tome. I don't care what kind of D&D game you play, there is something in this book that will make your campaign cooler.
If I had my way Dungeon Interludes would be the new template for how prepackaged campaigns are made. You get an epic series of six dungeon crawl adventures, spanning levels 1 through 13. But here's the catch: it's not a continuous campaign that will occupy your group nonstop. Instead, you are supposed to play at least one other adventure in between each chapter. So you get the continuity of an epic plotline without the shackles of being committed to slogging through the plot at the expense of all other adventuring. I used Dungeon Interludes filled out with Adventure I as the first plot arc of my last D&D campaign and the results were pretty effin' sweet.
Minions is just a goofy little monster book with no pretensions. A lot of companies make monster books that use the critter entries as an opportunity for fleshing out their official campaign setting. While there's nothing wrong with that approach, I prefer a grab-bag of critters that I can use as my own campaign building blocks.
When Iron Heroes first came out I wasn't ready for it, in terms of my own comfort working under the hood of the d20 system. Since then I've run several sessions of d20 modern and an 18-month D&D campaign. With this experience under my belt, I feel more prepared to tackle this crunchy D&D alternative. Iron Heroes takes a lot of the cruft that has haphazardly accumulated on D&D and trades it in for more sword & sorcery style badassery. Awesome-tastic. The only thing that this game lacks is in the magic department. If anyone knows of any good tips for combining Arcana Evolved's magic with Iron Heroes' sword fu, please let me know.