I'm not sure I could think of 5. As some others have said, I rately finish them if I don't like them:
1. Jonathan Strange and Mr Norrell. Good god it was dull. I managed to persevere but it was hard going. Felt like nothing actually happened.
2. Lord of the Ring trilogy. I understand why so many people love it, but it was just so loooong! Cutting out the songs and tedious descriptions would have done wonders.
3. The Suspicions of Mr Whicher. Completely unreadable. I managed about 1/4 of it before I had to give up.
I'll come back to 4 and 5!