Well, I don't know about the 'worst', but here's my 5 least useful...
First, my 5 least useful books so far are:
Dragon Lords of Melnibone (Chaosium) - Ok, first off, I'm not a big Moorcock fan, but I hear good things about all things Elric all the time, and the book looked good on the store shelf, so I bought it. So far, I've used absolutely nothing from it, not even a token feat, spell, item or monster. Just nothing there I found useful or interesting. It didn't read very well either, and didn't inspire me to go read Moorcock.
Nightmares & Dreams I (Mystic Eye) - Same basic thing here. Not one of the monsters has made an appearance in my game. I've not been able to get my mind around the whole nightmare world/real world concept either, but that's probably just personal taste.
Nightmares & Dreams II (Mystic Eye) - Note: After my opinion of the first book, I didn't buy this one. I won it in one of the ENWorld D&D3E celebrity chat trivia contests). Same thing basically, nothing here that seemed cool or unique enough to drop into my game.
Psionics Handbook (Wizards of the Coast) I'm not sure what exactly I was hoping they would do to psionics in 3e to make them cool, but this book wasn't it. It's sat quietly on my shelf untouched for well over 6 months, noone in my group wants anything to do with it.
Dungeons & Dragons Gazetteer (Wizards of the Coast) This book is absolutely, 100% useless. Even as an LGG lite it fails to capture any of the feel or style of the Greyhawk setting, and IMO is the reason there has been little or no interest in the Greyhawk line outside of Living Greyhawk. IMO, they should have dumped this book, taken Living off the cover of the LGG, and called that the 3e Greyhawk sourcebook. The mistaken assumption some people had that that book would be useless if they didn't play LG killed any real chance the setting had at competing with the more well promoted settings like FR, Scarred Lands, Kalamar, etc.
Now, I must comment on the bad rap the Hero Builder's Guide got. Again, it's just that Wizards marketted it wrong. As a source of ideas and inspiration for newbies, it did well. As a collection of random charts to do a 'quick fleshing out' of an NPC, it did well. As the definitive guide to developing a PC, it bombed. Given what it was actually intended for though, it does well, and I think the criticism it gets is unfounded. Blame the marketting department, not R&D.