More than 50 years ago, Isaac Asimov defined science fiction as "that branch of literature which is concerned with the impact of scientific advance upon human beings." For many that definition has evolved into a literature of consequences – of “what if.”
This art of “what if” characterizes sci-fi classics such as Ender’s Game by Orson Scott Card; Dune by Frank Herbert; Foundation by Isaac Asimov; Hitchhiker’s Guide to the Galaxy by Douglas Adams; and 1984 by George Orwell. Those five books seem to top many “best science fiction of all time” lists.
For a good guide to more recent sci-fi, try the SF Site. It offers everything from reviews and author lists to "Best Read of the Year" and “10 Odd Sci-Fi Classics.”
Sci-fi has a habit of predicting reality, whether it is Jules Verne’s “rocket” in his 1865 novel, From the Earth to the Moon, or the totalitarian regime in 1984. For this reason it endures.