I have to go with Orwell's masterpiece, 1984. I doubt there are books as influential and maybe even prophetic as this one.
The story is fascinating, intricate and incredibly original; the characters are extremely complex; but I think what the most impressive thing about this book is its concepts and ways of thinking. The views on propaganda and the manipulation of information is something that gives you a lot to think about, and don't even get me started on Big Brother.
Truly an exceptional book.