I've never read anything by Asimov, and I've only ever read 2001: A Space Odyssey and Childhood's End by Clarke, the latter being a book I was assigned to read in Advanced Placement English in 12th grade. I've read a lot of Heinlein, some Phillip K. Dick, and only a few Ray Bradbury.
I suppose it depends on what you mean by "Greatest" and "Writer," at least. I noted that none of the people on Google's list were movie writers, but George Lucas' Star Wars is arguably the single most influential scifi work ever created. Melissa Mathison wrote the screenplay for E.T. The Extra-Terrestrial and I don't see her on the list. So we don't include "screenplay writers" as "writers" in this category I guess.
I was in the ER for a blood clot just before Xmas, replacing my annual Xmas flu with something decidedly more potentially-fatal, but nonetheless easier to have. I told that to Sweetie, just that way: I said I'd rather have a blood clot in my leg that might kill me without much notice than the stomach flu because I'd been able to live my regular life with a blood clot, only slightly inconvenienced: I missed no work, went swimming with the boys, and did almost all my usual stuff including eating fancy doughnuts during our annual Xmas Shopping Day Of FunStravaganza (TM), while with stomach flu all you can do is sit around and wish you were about to die because then it would be over. This story has a point. The point is this: To kill (morbid pun intended) time in the ER, I brought with me Robert Heinlein's book Friday, which I'd read a long time ago and was re-reading as part of 2016's 100 book.
The blood guy who had to take a sample to see what drugs would keep me alive until my insurance ran out (with insurance: 400 bucks a month for one kind of medicine, just to stay alive.) He saw the book, with its provocative (slightly) cover, and asked what I was reading. I told him it was a scifi book about a sort of created-human who works as a courier and is trying to survive a weird sets of coups in what's left of North America. Who's it by? He asked. Robert Heinlein, I told him. Never heard of him, he said. The guy was about mid-20s. I said Stranger in a Strange Land is probably his best-known book. He shrugged.
Number 3 on the list of all-time writers gets you a shrug.
I think the list of all-time writers is weighted towards 'golden-era' writers, and weighted towards those guys we think are important. You have to go way down the list before you get to Larry Niven, Douglas Adams, and Kurt Vonnegut. China Mieville and David Brin are on there. Alan Dean Foster and Nick Harkaway are not. Why not? Too recent? To few books? Not deemed weighty enough? I would be willing to bet that Adams has sold more books than Isaac Asimov.
Has Isaac Asimov Sold More Books Than Douglas Adams? A Thinking The Lions Investigation:
Answer: Apparently, yes. Wikipedia says that Asimov's Foundation has sold 20,000,000 copies while Adams Hitchhiker series has sold 16,000,000.
By sales alone, Jules Verne should be our most important scifi writer: 20,000 Leagues Under The Sea is said to have sold 60,000,000 copies. The next closest scifi book is 1984, which has sold only 25,000,000 copies. That is closely trailed by The Hunger Games trilogy at 23,000,000 copies. As an aside, I would rather eat a copy of 1984 than watch anything with Jennifer Lawrence even vaguely associated with it.
I once argued that you could tell whether something was truly great by how many people had ever heard of the thing. Greatness, I posited, was something that was known rather than understood, and if people repeatedly talked about something, it was probably great. (The counter to that is lots of things that are demonstrably not great like Donald Trump and the Zika virus are also well known.)(The counter to that is this: would you rather be infected with Zika or have Donald Trump be president? Answer in the comments.)
(I would rather have Zika. To be honest I would rather that Donald Trump have Zika than be president, and I could just go on with my life, but I had to choose one or the other.)
So sales are actually probably a good measure of how great a writer was, as is the fact that people still know who Jules Verne is even though he died three centuries ago.
Did you know that Jules Verne was also a poet and playwright? Why is a play writer a 'playwright' and a book writer is an 'author?' Verne quit being a lawyer to write, launching the Voyages Extraordinaires series, which was said to be 'well-researched," and I guess it was for the 1860s-1870s and actually nobody has ever proven that there are not people living in the center of the Earth, so take that.
Apparently one problem with the esteem in which we hold Verne is that his novels were written in French and are frequently poorly-translated as kidlit, so we don't think of him as a big-time author. Verne himself argued that he wasn't writing 'science fiction.'
Heinlein's books are the sort of books that someone would hold in high esteem. They feel as though they are worth it. They're mostly long, very talky at times, have some hard science in them and some science that seems like hard science but maybe isn't? The first Heinlein book I ever read was The Number Of The Beast, which has a lot to say about non-Euclidean geometry and mathematics and computer science and random numbers, but also features busty blondes sunbathing naked and trips to other universes and the like. It was pretty good-- a good mix between Heinlein's too-talky works (Time Enough For Love, e.g., which I started to re-read last year but gave up on as too boring) and Heinlein's more basic short stories and scifi novels (like The Puppet Masters, which felt non-Heinleinian). I had picked The Number Of The Beast up off the shelf of the UW Bookstore back when I first came to school here as an undergrad.
Stranger In A Strange Land is the one people who know Heinlein or know scifi or both remember, really: the story of Valentine Michael Smith, a man raised by Martians and then brought home to Earth who starts his own religion is, as I remember it, a great story full of vague references to sex, discussions of art and politics, and, ultimately, a discussion of humor that somehow works its way to a cannibalism scene about religion. So it goes, to take a Vonnegut line.
A lot of Heinlein books are vaguely tied together: The Cat Who Walks Through Walls and The Moon Is A Harsh Mistress and Time Enough For Love and To Sail Beyond The Sunset all combine with The Number Of The Beast to present a multiverse in which various dimensions exist including those dimensions which are created as fictional characters: a character in The Moon Is A Harsh Mistress grows up to be in The Cat Who Walks Through Walls, in which she meets a previously-fictional character she helped write stories about one time. It's all pretty entertaining and has the feel of science.
"Pretty entertaining and has the feel of science" is how I'd describe Friday, which feels like it's set in that same universe but makes no reference to any of the other books. Friday is an 'artificial person,' a human created by gene splicing to be smarter, faster, and stronger than other humans. At the outset of the book she's returning from a mission as a courier, but soon gets sidetracked by a series of coups that result in border closings and trouble for her getting around on Earth (an Earth in which California is a democratic Republic, an area around Chicago is a tyranny, etc. and corporations are pretty powerful). Much of the book is devoted to Friday's attempts to report in, and then the latter part of the book is her first 'mission' as an independent person outside of her organization.
It's a good book-- less talky than a lot of Heinlein, more action, and the exposition works well with the story. There's not a lot of explanation of stuff given that a reader of a memoir by someone like Friday -- the book is presented as a memoir -- would need explained, which works well. By that I mean, if Friday were a real person and wrote a real memoir, a reader of that memoir wouldn't need Friday to describe political systems and what "Shipstones" are (a kind of battery) and so on. I like that kind of scifi: where the stuff is presented as "I don't really need to explain this to you because you know it," making the book move better and feel a bit more real. Luke Skywalker never stops to explain what the Senate was, after all.
Is it a great book, worthy of making Heinlein at least the #3 scifi author of all time? I don't know. When it comes to measuring greatness, it's not enough maybe to determine a writer's worth simply by how many books he's sold, or how many books he's written. Maybe we have to take that into account but also measure his impact on the culture -- not just by the esteem we grant him (or any writer) but by how the culture pays tribute to him.
Heinlein doesn't appear on that Wikipedia list of best selling authors. OMNI Magazine ranked him 10th (below, among others, Ursuala K. Leguin) but said he's the "Dean Of Science Fiction" and quoted Asimov saying that Heinlein was 'the best science fiction writer in existence.'
Heinlein seems to be revered for popularizing scifi (he was 'one of' the first to get a scifi story into popular magazines) and making social commentary in scifi commonplace. Someone founded a religion based on his Stranger in a Strange Land. Astronauts credited him with making a trip to the moon seem possible and he was a guest commentator for the Apollo 11 trip. He popularized some terms, like "Pay it forward" (which appears, in philosophy at least, in Friday.) He was embraced by libertarians and hippies, but criticized for appearing to accept pedophilia and incest. Elon Musk says Heinlein was an inspiration. He is in the Hall of Famous Missourians along with Scott Joplin, Dred Scott, Bob Barker, and Betty Grable.
He also had a talking rat character named after him in a movie. So it goes.