Practically the first item I saw when I went online a few minutes ago was the death of Arthur C. Clarke. He leaves a vast and influential body of work behind, books like Childhood's End and Rendezvous with Rama and Prelude to Space and Earthlight...oh, and he's partially responsible for a little thing called 2001: A Space Odyssey.
He's also largely responsible for the communication satellite, being the first person to work out the appropriate 24-hour orbits they would have to take up to be effective.
By default, on the departures of Heinlein and Asimov, he held the title of "Greatest Living Science Fiction Writer." The field has expanded so much of late that I couldn't say who succeeds to the title.