My favorites writers in sci-fi are Ray Bradbury ('The Martian Chronicles' and 'Fahrenheit 451' are probably the best books I ever read, in my opinion) and Stanislaw Lem (someone could remember 'Solaris', and he has many other fiction books and studies in futurology).
And many other brilliant authors - Robert Sheckley, Kurt Vonnegut, Clifford Simak... Herbert Wells and Jule Verne (not sure about spelling).
I love science fiction. I also re-read for several times books written by Arkady and Boris Strugatsky, Isaac Asimov, Ivan Antonovich Efremov, Arthur Clark and so on.
Recently I have read three books written by William Gibson, including 'Spook Country'. I also like them, but it feels to me I prefer traditional sci-fi anyway.
Wikipedia entry lists Franz Kafka and Edgar Allan Poe as science fiction authors, but I am not sure. I like their books too, anyway.
If I need to select the only one, it would be 'The Martian Chronicles'. It's wonderful written and made me to think about vital and 'higher' questions. These short stories are not necessarily connected with each other. They look rather like a book of poetry.