Worst book ever read is really tough, I read so many books, and I am sure there are bad ones I have blocked out. Books that I distinctly remember disliking include:
Cold Mountain. I actually threw this book across the room because I hated the ending so much.
Divine Secrets of the Ya Ya Sisterhood. Total treacle, and so many people really liked it, which added to my disappointment.
Books that tortured me in college: The Faerie Queene, Old Grizzly.
Finally, a special mention for Gerald's Game by Stephen King. I was reading this book in the kitchen, got to a particularly grotesque part involving attempted escape from handcuffs (details omitted!), and actually passed out, fell backwards, hit my head on the floor, and ended up with a concussion!
(but I finished the book anyway)