The Brothers Karamazov in my case. I got half through it twice. Its a great book, but man, its a chore to read and follow casually.
War and Peace, on the other hand, I thoroughly enjoyed (well, not too crazy about the last 50 pages). Need to read it again.
Crime and Punishment is a great read. Fathers and Sons isn't bad.
Has anyone else read David Baldacci's The First Family? Published in 2009. The President and First Lady in the novel seem to be based in large part on the Clintons. (Not as bad, of course.)