Thinking about it, the worst book I can recall finishing was Eoin Colfer's Artemis Fowl, which I read about ten years ago. Its antagonist, Artemis Fowl, was described as prodigal genius, yet none of his actions actually demonstrated that he was one. Its plot was unmemorable, especially since Artemis was supposed to be some sort of criminal mastermind. Its other characters were one-dimensional - they had maybe one trait and that was that. Its humor lacked wit and the action lacked excitement; both seemed really contrived. But really, what bothered me so much about it was that are actually a lot of good books aimed at preteens that are much better and that I could have read instead.

I think the second-worst (at least right now) book that I've finished is Anne Rice's The Witching Hour, mainly because it was a 1000 page slog with an unsatisfying ending. It wasn't really scary, it wasn't really insightful, and mostly it had me feeling grossed out. Granted, the writing itself was fine.

I hated Stephen R. Donaldson's Lord Foul's Bane, but I didn't finish it. Also, some of the Thieves' World stories were really awful.

As far as Neil Gaiman is concerned, I liked Anansi Boys, American Gods, and Sandman (at least the first 2 volumes, which are the ones that I've finished) a lot. I wasn't as crazy about Neverwhere, but I wouldn't call it one of the worst books I've read.