Top five favorite:
The Stand by Stephen King
The Hobbit by J. R. R. Tolkien
Artemis Fowl series by Eoin Colfer
Percy Jackson and the Olympians series by Rick Riordin
Bag of Bones by Stephen King

I don't really have any least favorite books I could put on this list because every book I've disliked, I've stopped reading halfway through. Except for The BFG by Roald Dahl. I had to read that in elementary school once.