Sorry I'm late to the thread.
I love sci fi books in general, but I think the best ones are the classics--I'm talking LeGuin, Huxley, Bradbury, Orwell, Asimov. I really appreciate the cautionary tales of the future, the struggle of man vs. technology, the things we lose (identity, security, community) in the service of scientific advancement...more than any other genre of fiction, sci-fi takes a critical look at humanity and the direction it is taking.
My favorite is "Frankenstein" by Mary Shelley. It's the best (albeit fictional) examination of ethics in science, and the consequences of ambition and curiosity. And it has the best line ever spoken by a villain: "I have love in me the likes of which you can scarcely imagine and rage the likes of which you would not believe. If I cannot satisfy the one, I will indulge the other."
For those interested in my Top Ten:
Fahrenheit 451
10. "20,000 Leagues Under the Sea" by Jules Verne
9. "The Foundation Trilogy" by Isaac Asimov
8. "Journey to the Center of the Earth" by Jules Verne
7. "The Time Machine" by H.G. Wells
6. "Brave New World" by Aldous Huxley
5. "Fahrenheit 451" by Ray Bradbury
4. "I, Robot" by Isaac Asimov
3. "Slaughterhouse Five" by Kurt Vonnegut
2. "The Left Hand of Darkness" by Ursula LeGuin
1. "Frankenstein" by Mary Shelley
Honorable Mention:
"Flowers for Algernon" by Daniel Keyes
"Cat's Cradle" by Kurt Vonnegut
"The Martian Chronicles" by Ray Bradbury