Tough question . . . Bradbury definitely deserves a place. And Ursula Le Guin and Samuel R. Delany, too, both for academic as well as fiction contributions, and Delany for breaking a few genre barriers in the 60's and 70's.

Or you could go back earlier, for Philip Jose Farmer, who's still alive and kicking and who was the first guy to put sex in SF in 1952, which changed the genre forever.