I'd have to go with Book of the Righteous. Even if you only use bits and pieces of it, the sheer amount of info is worth every penny. And the "Holy Warrior" class is the paladin done right!!
I agree. And what's super cool about BoTR is how is how other GR books fit right into it, even though none of them are co-dependent. Legions of Hell, Armies of the Abyss, and the Avatar's Handbook all work so well with BotR, and presumably the Unholy Warrior's Handbook will too.
For my home campaign, it's a tie between the Forgotten Realms Campaign Setting and d20 Modern.
For the purposes of writing professionally, I have to say Mercenaries and Oathbound.
It's gotta be the DMG. There's just so much useful stuff in there, for players and DMs alike.
Players can compare themselves to the NPC characters to tell whether they're about the right power level, DMs can do the same. It's got basically everything you need as a DM, and I've no complaints with it at all.