Brave New World, by Aldous Huxley - This is in close competition with A Clockwork Orange as my favorite book. The message that this book conveys is one that sums up a large part of my personal philosophy very well. If you love dystopian sci-fi, or books that make you think, I highly recommend this one.
Brave New World by Aldous Huxley
A Clockwork Orange by Anthony Burgess
Among the Hidden by Margaret Peterson Haddix
Ender's Game by Orson Scott Card