It will sound horribly cliché by now, but yes, Twilight is the worst book I have ever read.
Honourable mention goes to "The Pillars Of The Earth" by Ken Follett.
My god, what tripe. This man took an awesome concept and completely butchered it. He can't write for shit. I'm interested in the mini-series though, since the whole thing reads like a movie script anyway.
This "My Immortal"-thing that gets mentioned every so often, I looked it up, and it even has an entry on tvtropes. I'm half-tempted to go read the whole thing, but I'm afraid I might actually die laughing. "I jumped sexily in front of da bullet" indeed.