Best:
Players Handbook II: Absolutey the best book this year, maybe even the best supplement for 3.5 (tied with the Expanded Psionics Handbook.) The new Fighter feats, the variant class abilities, and the Duskblade (!!!!!!!! best class ever!!!!) make this almost the perfect D&D supplement.
Complete Mage: Reserve feats, awesome Prestige Classes, and some rad spells. I really liked this book. It makes me feel very optimistic about the second run of Complete books.
Worst:
Complete Psionic: After the complete lack of anything for psionics, the intense anticipation for this book, we get this... steaming pile of flop?! There is one sole redeeming feature of this book, and it is the Ardent. Other than that, the feats are horrible (how many feats did we need to allow a Soulknife to mold his mindblade into another weapon, anyway? And Illithid Heritage feats? Illithid do not breed with humanoids nor do they have humanoid descendants!) the power list includes ERRATA for the XPH (absolutely the most horridly blatant padding of a book I have ever seen) and scads of useless powers, and it has completely uninspired and insipid Prestige Classes. After waiting so long for more psionics material, this was like a slap in the face from WotC for psionics fans. Unfortunately WotC will probably look at the poor reception of this book and decide that it's because psionics don't sell, and ignore the atrocious quality of the book as the reason for it's failure.
Struggling to like:
Tome of Battle: the Book of Nine Swords: I want to like this book. I really really do. I love the flavor and the inspiration behind it. But it really feels like the writers just decided to take the Fighter class out behind the wood shed and put a bullet in his brain with this one. Absolutely no balance between the Warblade and the Fighter. It's like someone actively tried to make the most overpowered melee class imaginable, giggling the whole time ("and we'll give it D12 hit dice, and 4 skill points per level, and Weapon Specialization even better than Fighters get! On top of its magic abilities! hehehehe!") and succeeded magnificently. Apart from that it's OK. I love the Swordsage, but even then I can't stop comparing it to the hellishly overpowered Warblade and it seems anemic in comparison. The Crusader makes me yawn, and the random availability of its class abilities makes me wince.