I have to agree with Bill -- Bradbury's was/is the best-known science fiction writer. But I think I'd have to qualify that a bit, and add "to people who actually read books" (and how many do that anymore).

To the average kid-on-the-street playing some videogame on his I-phone or sending yet another text message, the most famous "sci-fi" writer could be someone like that guy who makes all those space-battle movies.