Very few novels ( and no great ones ), but reams of short stories ( some of them superb ). Also, of course, tons of criticism, screenplays and essays. He’s actually pretty prolific, but works outside of strict sf frequently.
Not my favorites, but I think for historical impact either Jules Verne or H. G. Wells has to get my nod.
Asimov just ahead of Heinlein.
Asimov with Heinleiin a very close second.
L Ron Hubbard
I have always found Heinlein’s early work kinda juvenile and his later work overwrought. I thought about some lesser-known writers who might deserve mention. I thought about it and decided that the standard for me would be which ones were the ones who set my imagination afire. Andre Norton was one, with her early stories of alien spaceships programmed to go on a tour of empty spaceports from their long-vanished civilization (Galactic Derelict) and terror weapons from long-dead wars luring spaceship crews to their doom (Sargasso of Space) or space patrols from dying empires looking for a place to settle during the the interregnum … she painted a vivid future of humanity agaisnt a big galactic backdrop. Then she went off on the whole fantasy thing. Sigh. Witch World was good but that was about it.
A. E. Van Vogt wrote some stuff that really fired me up … most especially The Voyage of the Space Beagle, that was a really vivid story of alien worlds … I was too young when I read it to get the reference in the title, just thought it was terrific fun …
But the guy who wrote the most mind-blasting stuff for me was Asimov. The Foundation Trilogy was the most powerfully expressed Galactic Empire I had encountered to date … it wasn’t just spaceships and mutants and advanced science, but the concept of human beings living on a scale that just dwarfed anything we now had … Asimov’s galactic capital of Trantor filled my head with visions that would never see the light of day until Lucas realized them with his galactic empire city in Star Wars Ep IV. The Caves of Steel presenting people to whom hallways and apartment rooms were their natural habitat, just as forests and farms were the natural habitat of humans for most of history … his robots as artificial minds controlled by strict laws of programing … oh, he had a way of reimagining human life that was just mind-blowing.
I might also consider Iain Banks, he comes late to the game, but his Culture civilization is the most powerful SciFi creation I’ve seen in decades.
But overall, I’d say … Asimov. Yeah, Asimov.
I note that no one has nominated Spider Robinson for the poll. I would like this post to be an anti-Spider Robinson vote, such that it requires two votes for him to bring him into positive territory.
As far as nominating someone for inclusion, the low-hanging fruit seems to have been plucked already. I’m going to suggest the one-hit wonder Walter M. Miller. You know, just as a dark horse.
We’re watching you.
This thread is just to get a list of candidates for the poll, right?
I was coming in here to nominate Spider Robinson anyway; this just made it necessary instead of a mere pleasure.
I would also like to second an earlier nomination for John Varley. Also, I may be the first to recommend Joe Haldeman.
Walter Miller, Stanislaw Lem, Phillip K. Dick and Frank Herbert were all light years ahead of Heinlein was users of language, characters and situations. C.J. Cherryh belongs up there too. Heinlein, Asimov and Ellison are complete hacks writing pulps by comparison. Not that they don’t do enjoyable things, but the aforementioned wrote literature (Cherryh is still alive and writing).
Ursula K. LeGuin is also a damn good writer, but let’s face it, her stuff isn’t any more science fiction than Ray Bradbury’s. She is a fantasy writer, and one of the best that ever lived.
Gene Wolfe.
Maybe so, but so were plenty of other writers who never wrote a lick of science fiction. The question was “who is the greatest science fiction writer?” not “who is the greatest writer who wrote science fiction?” I think there’s a distinction.
And, I don’t think one-hit wonders are really in contention. The question also wasn’t “Who wrote the greatest science fiction?” I don’t think we should judge writers solely by the greatness of their greatest single work, but by their whole body of work.
Ursula K. LeGuin gets my vote, which I’m sure is a big surprise :D. Heinlein gets my anti-vote, but he only barely edges out Asimov. Both of them are fine for story (not great, just fine); Asimov seems to have a pretty good head for the science; but their prose is horrible and their characters are ridiculous and often interchangeable. Heinlein gets the vote because of his mouthpiece characters. I really don’t like Heinlein’s work at all.
My favorite recent author is Robert Charles Wilson: Spin is the best SF I’ve read in many years.
Isaac Asimov.
He says he’s “never written science fiction” (first mp3 here, less than 2 minutes in: http://www.studio360.org/episodes/2009/05/29). I suspect his approval of the contrarianism would be outweighed by his irritation at the label. Irritating him is fun because it’s so easy, but it has considerable risks, too. 
This. Couldn’t have said it better myself.
What he said.
I’m tempted to vote for J.G. Ballard. As a creator of concepts, I think he is unequaled. But philosophically, he is extremely negative. Reading his work is emotionally draining; he’s almost like the George Orwell of science fiction.
Ahem.
It was the very first reply.
Tie between Heinlein and Philip K Dick, maybe with Heinlein ahead by a nose.
Yeah, but his middle work! The Moon Is A Harsh Mistress, Starship Troopers, and Double Star are really excellent. Stranger In A Strange Land, too, though he’s edging into his talky phase. Not to mention some really great shorts, like By His Bootstraps, All You Zombies, The Menace From Earth, The Roads Must Roll, Life-Line, and so on. And if we’re talking about “greatest” as “largest contribution to the field”, then we need to give RAH some credit for sucking John Varley, Larry Niven, and Spider Robinson into the science fiction world, along with countless other writers who got into science fiction because of RAH’s juveniles.
You can’t go wrong with the Good Doctor, and by the above comment about “largest contribution to the field”, Asimov deserves almost as much credit for establishing the field and for creating a new and impressive magazine dedicated to science fiction. Isaac Asimov was a significant ambassador for the field, and the only reason I’d vote for Heinlein over Asimov is that Heinlein was equally impressive in short fiction as in long fiction, while the only Asimov novel I’ve ever really enjoyed was Foundation, and not its sequels.
Oh, and David Brin deserves a mention too.