As a player:
Player's Handbook II, Spell Compendium, and Magic Item Compendium. Assuming your DM will allow those materials, they'll expand your choices a fair amount and, in the case of PH2, improve the power of fighters and such to be less significantly disadvantaged (though it's got some useful stuff for spellcasters too, of course).
As a DM:
Lords of Madness, Cityscape, and Dungeonscape are probably good choices. I dunno about the last one though, it may or may not be more worthwhile than the Dungeon Master's Guide II.
Other:
Overall, really, I'd suggest the Tome of Battle: Book of Nine Swords, Lords of Madness, and either Draconomicon or Player's Handbook II or Monster Manual III. This covers a broad spread of player material and DM material, and adds some coolness to the game.
The PH2 is not, however, quite as useful if you get the Bo9S, since PH2 has some material that helps Fighters become stronger, while the Bo9S makes Fighters rather obsolete and boring by comparison (not useless, just comparatively less effective, since Bo9S classes are warriors that nearly match spellcasters in power; they're slightly weaker than spellcasters, but can use their cool attacks in every battle, so they have more endurance but less oomph; also, Bo9S classes kinda suck at range, since they have access to only a handful of kinda-minor ranged attacks, and little if anything that helps a bit with ranged weapons).
Fair warning though: apparently many people who dislike anime also hate the Book of Nine Swords because some parts of that book have an anime or wuxia style to them. Some parts though are quite suited to standard D&D folks like knights, swashbucklers, or holy warriors. If you don't mind anime or heroic, over-the-top action in D&D, then the Book of Nine Swords should be a good investment. It makes warriors really fun and interesting to play.