I didn't read my Sisters' Keeper - I saw the movie and cried the whole way through. Anyway, in the movie Kate died at the end - did the book end differently?
My vote for worse book ever - The Girl who loved Tom Gordan by Stephen King. It's about a little girl lost in the woods. Halfway through I was praying that a bear would eat her in order to liven up the plot.
I read that book on the recommendation of a coworker who *loved* it. And, honestly, I really enjoyed the parts that were about her survival in the woods. It was the spiritual bear thing... was it supposed to be the devil incarnate or something? I forget. Anyway, it was that whole thing that just ruined it for me. It just didn't fit the story.
It reminded me a little bit of a romance novel I read once, a Regency romance novel, in which the antagonist was... the devil. Yes, the actual devil. He was in human form messing with people for the fun of it. It was like the author couldn't come up with any reason why somebody would be trying to hurt the main characters, so she made him the devil so he'd need no reason.
Now, I could totally see that plot if the devil character was introduced at the beginning and it was established early on how he messed with people, etc. I've read books that use angels or otherwise to that effect. But when you have an otherwise typical Regency romance and then at the end you find out that the antagonist is the devil? It might induce gales of laughter, but not make me want to read your other works!