It has to be almost any of the Mr Men books, for me. They are unreadable. I seriously doubt that anyone bothered to read them aloud at any point during the editorial process (a criticism that could be leveled at any number of books published today too.)Reading them aloud is like the verbal equivalent of walking barefoot over broken bricks.
Come on, dish the dirt. Which ones really make your heart sink when your child hands them to you to read out?