The books I've read the most:
Snow Crash, which is pretty much the funniest sci-fi book I've read, which also includes some of Stephenson's most unusual and hilarious implications about ancient historical events.
Good Omens, pretty much the perfect combination of Pratchett's wit and Gaiman's dark alternative universes.
Small Gods, also Pratchett, and probably my favorite Discworld novel.
Catch-22, not just the penultimate anti-war book, but also a comedic masterpiece of insanity and violence.
What? All humorous books? (OK, Snow Crash isn't STRICTLY humor, but it's so tongue in cheek there's practically a hole in his face) Alright then, I'll go with House of Chains from Stephen Erickson's Malazan Tales series. Even knowing the outcome of the story, I simply am amazed at the development of the main character in this book. It's an epic journey that I think none could predict, even if you knew what happened to the character in the end.