My all time favourite would be Blood Meridian, but very close would be A Canticle for Liebowitz, A Clockwork Orange, and Slaughterhouse 5. All brilliant books, and the last two are very short as well.

Canticle hits my post apocalypse and medieval loving brain in exactly the right spot, Clockwork is so challenging at first but the immersion is ridiculous, and Slaughterhouse is a brilliant piece of unreliable narrator fiction.