How can you ask me to pick just one? There's The Hitchhiker's Guide to the Galaxy, The Long Dark Tea-Time of the Soul, The Cost of Discipleship, The Great Gatsby, The Great Divorce, Do Androids Dream of Electric Sheep?, and a buch more.
If I had to pick just one, I would pick Crime and Punishment by Fyodor Dostoevsky, and A Tale of Two Cities by Charles Dickens.
Yes, that was two. You can't make me choose. 😛