I would like to vote against Heinlein. I’ve read three of his books, all boring repetitious crap. I don’t care how much you say “but all his other books were great!”. He wrote at least three that were crap and with all the books he wrote the odds that I would chose the only ones that were crap is unlikely, so by that I surmise he probably had a lot more crap.
Hubbard loses for starting a whackjob cult, but his books weren’t good enough for him to even be in the running.
Heinlein, of course.
But I’d like to throw some dark horse nominations out for more recent writers. David Brin and Jack McDevitt both write spectacular SF in the ‘let’s explore and see what the universe is like’.
Brin, in particular, works to get the science right and does the ‘what does this imply’ thing well.
Sturgeon*'s Law: “Ninety percent of everything is crud.”
*Another fine science fiction writer, already nominated upthread.
Maybe not the greatest ever, but I think Poul Anderson should be included in the list. A very prolific writer who wrote very enjoyable Science Fiction (Tau Zero, and The High Crusade to name two of my favorites).
I beg your pardon - that was not one of my brightest moments. Don’t ask me how I interpreted that to mean the avatar was not being voted for.
Heinlein with Asimov as first runner-up.
And Frederik Pohl as Miss Congeniality. 
I don’t read science fiction for the quality of the prose. I read it for the quality of the ideas and the stories.
I cheerfully concede that Heinlein and Asimov wrote some clunky-ass prose. Yup yup. No Nobel Prizes for literature for those two. and I really don’t care.
He wrote quite a number that were crap, especially later in his career. There was a point where he kind of went off the deep end, and wrote a lot of, as you say, boring and repetitious crap. But that has no bearing on the quality of the work he produced in the early and middle parts of his career.
Which ones did you read anyway?
Actually, Ira Levin deserves a passing mention, most notably for This Perfect Day. While not a stellar work of fiction by any standard, it’s still one of my favorites 30 years after having first read it. One of my favorite SF themes has always been rebel-in-a-totalitarian-society, and that one is hands down the best of the bunch.
Has anybody mentioned Delany yet?
Sorry, I saw that as more of a footnote than a nomination. And I totally missed Le Ministre de l’au-delà’s post.
I’ll have to recruit a second anti-Robinson nomination, I see. I just can’t forgive him for the self-indulgent, juvenile, wish-fulfilling crap he pulled when he had Michael Finn make all the regulars at Callahan’s magically impervious to the effects of nuclear explosions.That’s the kind of writing I outgrew when I read my last Bobbsey Twins story, ferpetessake.
It doesn’t reflect any credit on RAH to have the likes of him be head cheerleader.
I wouldn’t really consider him a science-fiction writer. Even The Metal Monster is still much more firmly in the lost-world adventure fantasy genre.
Definitely one of my favorite authors, though.
Why would anyone mention that putrid hack?
A lot of my friends were into him when I was younger. I tried to read one of his works—a long, strange book called Dahlgren, but dropped it about 80% of the way through. I’m not sure how much of a following he has, though.
Heinlein. Not just for his writing, but for his influence. Other writers wrote good books that people enjoyed, but Heinlein changed people’s lives. Something about his writing touched people right in their hearts.
Heinlein is responsible for an awful lot of kids going into science and engineering.
Jack Vance may not be the greatest science fiction writer ever, but I would re-read a Vance book before I would read anything new by most of the writers that have been mentioned.
As to Heinlein, I always imagined him living in a universe where editors were unknown. And everyone answered to the name “Dear”.
I can’t forgive him for a lot of that self-indulgent crap. His major characters have everything turn out RIGHT for them. One of the regulars at Callahan’s used to think that he was responsible for the deaths of his wife and kid(s), but Robinson retconned that to make it the fault of someone else. When he believed that he was responsible, he had some pretty good character development. Robinson could never have written the proper ending for Podkayne, and in Stranger, Smith would have not only lived, but everyone would have realized how RIGHT he was almost immediately. Until Robinson learns to let bad things happen to some of his favorite characters, and not give everyone that he likes a happy ending, he won’t be a decent writer. In the most recent Callahan universe book that I read (don’t remember which one, but I do remember that I was glad I’d checked it out from the library rather than buying it), all of the “good” characters are just fine with aliens, and people with different tastes and lifestyles…unless a person happens to be short, fat, and have bad taste in clothes. Then it’s OK to mock that person and give them shit. I also disagree, very much, that all the problems in the world or universe can be solved by getting high/stoned and screwing. Robinson can write well, but he won’t inflict damage on his darlings, much less kill them.
Personally, I think that Heinlein would have to win this thing. He wrote a lot of the common stories in SF, and either he wrote them first, or he wrote them best, or both. Yeah, he wrote a few stinkers, and he liked incest a little too much, but overall, considering both what he wrote and his influence on the field, I think he deserves to be known as the best. I do have other SF writers that I love, but Heinlein’s juveniles, along with Norton’s SF, got me through my childhood and adolescence.
I came here to say Jack Vance, possibly the most literate and elegant and imaginative writer ever to pen a science fiction tale.
But since he’s already been mentioned, I’ll nominate Roger Zelazny for his depth, breadth, and scope.
For me, it’s got to be Asimov. His work was profoundly influential on my life.
If we’re talking beautiful writing, I’d go with Cordwainer Smith for the consistent excellence of his work. If we’re talking innovation, I’d go with Weinbaum over Smith, because real aliens are harder to conceive of than spaceships - and because Smith’s writing and characterization are so awful.
But all in all, I’ll go with Clarke. He got me into engineering and computer science. He has more range than Asimov and or Heinlein, going from nuts and bolts stuff like Prelude to Space to transcendent stuff like City and the Stars and the end of Childhood’s End. Not to mention being partially responsible for my favorite movie. And while we are not judging science writing, Clarke’s is far superior, if not as prolific. Asimov was good at a superficial and tutorial approach to any subject, but Clarke was a far superior prophet, with far more imagination. (He says looking at his satellite dish.)
Very much like Zelazny, and glad he’s been mentioned. Greg Egan is probably the best science fiction writer of the last 20 years or so.
Delany’s talent for descriptive writing is, I think, unequaled. Forget Hogg - I think his best work is probably The Mad Man. I think it would take nothing less than a bonafide genius and true polymath - a genius of literature, philosophy, and aesthetics - to create a novel like that. I have also had many chats with Delany and he is an exceptionally clever, funny and kind man - though none of those things, of course, necessarily make someone a good writer.
I haven’t read enough of his science fiction or fantasy to vote for him in this thread, but he’s definitely prolific and very imaginative.
ETA - I haven’t read anything by Zelzany, but Delany likes him a great deal, for what it’s worth.