Harry Potter. Never before has a book enthralled me as much, made me fantasise about the world, the characters and the story. Provided hours of speculation and theories. Made me count down the days until the next release, wait fervently for any news regarding the next book and provided the will to dash to the store at midnight or release day. No other book has provided both the enjoyment whilst reading and the level of anticipation before release. It was the epitome of escapism.
Other books that make the cream of the crop would be the Lord of the Rings, His Dark Materials, Crime and Punishment, The Hitchhiker's Guide to the Galaxy, Dune, Foundation and 1984.