Hmm, this is really hard.
I'm so torn for the best book, I have to separate it into categories for best rules and best flavor.
Best Flavor: Sharn, City of Towers. While the Eberron Campaign Setting (3.5) was obviously what got me into Eberron, the Sharn sourcebook pretty much stole my heart. I've run in and played games set entirely in Sharn, and in one game where me and friends were playing private investigators, we once went about 6 hours (real time) of just pure roleplay, getting into the setting and our characters. The crunch bits toward the back aren't much to talk about, but the setting of the City of Towers is fantastic and gritty and noir and so, so awesome.
Best Crunch: Tome of Magic (3e). This book stands out to me as a sort of masterpiece theater of sourcebooks, detailing three entirely new magic systems, each with its own class, prestige classes, monsters, and other crunch. It was fantastically laid out, and it sparked my imagination so well.
Worst stuff. Hmm. Also hard to pin down. There are lots of books I never got to use (Tome of Battle, Epic Level Handbook, Deities and Demigods) but I derived lots of enjoyment from anyway (the ELH was just fun for me to read, and Deities and Demigods was like number porn. Just got a kick out of seeing what Thor's attack with Mjolnir looked like!). But if I had to choose one...
Dragonlance Campaign Setting (3e). It just did nothing for me. It described a world much better described elsewhere, and the crunch was laughable (Hey, wanna play a bard that has no spellcasting and gets nothing to make up for it? How bout a class even weaker than that? Then the Noble is right for you! Want to play a Paladin of Paladine? Tough! They don't exist!) Overall just a worthless book.