I'm personally going to say that Book of Exalted Deeds is probably the single worst WotC book I've ever read. It's even worse than most Pathfinder books, and is mechanically less sound than all but the worst Palladium or WW books.
I mean, a book for goody-two-shoes is interesting, sure, but seriously, the "Drugs and poisons are evil. Here, have Goodtm brand Poisons and drugs, and, hey, we'll throw in a mindrape free" idea was absurd. It's basically an even worse version of BoVD, but for the good guys. Compounding on this is the fact that I seriously cannot imagine Exalted being sustainable. It's just too strict.
Also, I'd like to nominate Rifts. The whole series. It's combat system is worse than Spawn of Fashan's, and the balance is worse than the worst White Wolf I've ever read.