ToB is probably the best 3.5 book out there. My swordsage leveled yesterday and I was reminded of how cool it was as I chose her new maneuvers. Fun classes, cool flavor...just a job well done.
I really liked Dragon Magic - the dragonfire adept is fun, the dragontouched races are cool, and the prestige classes are nifty. It mixes well with Races of the Dragon.
OH! And Dungeonscape is actually my favorite 3.5 book! Solid info for DMs, a pair of nifty PrCs, and some cool feats and alternate class features. Great stuff.
Also, as a GM I liked the first half of the DMG II - good, solid advice on DMing.