Asimov's two big universes, found in the Robots series and the Foundation series, (which turned out to be the same when he revisited them in the 70's and 80's) are simply wonderful. By far my favorite sci-fi, and among the most enduring works in the genre. I read them both every two or three years, and sometimes grab one off the shelf when I just want something to read. The early Foundation is sort of a future history version of "The Decline and Fall of the Roman Empire" (I seem to recall that was intentional, actually). Asimov's autobiographies and the posthumous book of his correspondence are all fantastic and insightful. I learned a lot from them, and my admiration for the man increased even more upon reading them.<p>Dune is also astoundingly good, though after the first three, the quality goes downhill (slowly, but surely). Definitely lots to think about.<p>Rendezvous with Rama left me breathless...and I could not put it down until I'd read the last page. I've returned to it a couple of times, and loved it every single time. The later Rama books are deeply flawed, and should have never happened (they were not written by Clarke, and it shows), as they make Rama seem...hmmm, I can't really think of the right word that expresses it. Cheap, dinky, small, something bad. But Rendezvous is a must read for everyone.<p>Douglas Adams, of course. The whole Hitchhikers' series is a must, while the Dirk Gently stuff is also funny, but not sci-fi and not mandatory.<p>I mentioned in another thread my love of dystopias, and one of the best ever is The Handmaid's Tale by Margaret Atwood. She's one of the best sci-fi writers alive (she doesn't exclusively write sci-fi, however). Oryx and Crake is also fascinating.<p>Cat's Cradle by Kurt Vonnegut. I enjoy all of Vonnegut's work, but Cat's Cradle is a magical perfect novel.<p>I don't know if everyone would agree that it's sci-fi, but Mark Twain's Letters from the Earth was an eye opener for me when I was a kid (I went on a Mark Twain kick when I was in elementary school...no one ever told me that all of his books weren't kids books, so I read everything I could get my hands on after being hooked by Tom Sawyer and Huckleberry Finn). Turns out Mark Twain was seditious, heretical, and quite the troublemaker all around.