Arcana Evolved. By far.
Although, that's mostly for the same amount of base classes and races as the original full sets of Complete and Races books, so you may consider that more of a 'player' thing. Personally, I consider it vital just for the AE giant, one of the coolest races ever for standard fantasy.
Otherwise, I'm not sure what you're looking for. A 'campaign in a box?' Shackled City, or Age of Worms when the hardcover comes out. A setting? I'd go with Iron Kingdoms, Eberron or DragonMech for something different - or Arcana Evolved itself, for the high fantasy angle. A book of house rules? Unearthed Arcana, DMG II, PHB II and Grim Tales are all solid choices, but the last will require the most work. Generally valuable material to flesh out your game? Expanded Psionics Handbook, Tome of Battle: Book of Nine Swords or, again, Arcana Evolved.