The Last Temptation of Christ. I've tried several times to pick it back up and I just can't get into it.
Then there's Finnegan's Wake. I'm not even going to try again.
That was a tough one. I almost gave up several times. I cannot make myself care about the Brienne character, and the book practically revolves around her. It's odd too, because the last hundred pages or so are awesome, and Dance with Dragons is a definite step in the right direction.