Starship Troopers with it's hard military science fiction. It gives interesting new outlooks on philosophy and morals too. Coming second is 2001:A Space Odyssey. I had to read the kiddie version first as it was pretty damn difficult for me but the full one was a damn fine book.
EDIT:Gasp! How could I forget Fahrenheit 451? It's a pretty good book too. The first book I finished with a craze for months. It broke my lazy-cycle.
EDIT:Gasp! How could I forget Fahrenheit 451? It's a pretty good book too. The first book I finished with a craze for months. It broke my lazy-cycle.