Hmm. Tough question. Let me see.
*peruses his collection of novels from Chrichton, Asimov, Tolkien, Rowling, Herbert, King, Verne, Wells, Clarke, Sagan, K. Dick, Vinge, and others
Nope. Can't pick a favorite. They're all amazing. The sad part is, even though some of the best science fiction and fantasy writers are also some of the greatest writers in history, most don't get that kind of recognition. Namely, because far too many people view those genres as a childish or "nerdy".
But you know what? To hell with those people. Some of the greatest stories I've ever read were based in science fiction and fantasy.