For me, it would have to be Ayn Rand's Atlas Shrugged. I don't entirely accept her ideas, and I think the book itself was barely above mediocre, but no other book has so drastically challenged my beliefs.
I used to be aggressively liberal, but now I find myself agreeing with conservatives more and more. Greed is virtuous, and welfare is armed robbery. I can't argue with her there. Atlas Shrugged transformed me from a bright eyed anarcho communist, to a cynical anarcho-individualist.

