Anything by Hawthorne or Hemingway. I love older books, but seriously those two authors are the hardest to read I have ever seen.
Most Boring was "Not Exactly the Three Musketeers" by someone I can't remember. Imagine a fantasy story where none of the character's backstories are explained; several references to the real world come up, but are never explained; and the main characters are completely unlikeable. I read to the very end, hoping that it would redeem itself, but it never did. In fact one of the three main characters dies and his death evokes no emotion because he was so unlikeable.
Oh, it also had a thing with feces. Yes, we know people evacuate their bowels when they die, you don't have to make a point about it every time someone dies.