Fame is only fame if you’re known to people outside of a normal fan base. That so many people professed never to have heard of Neil Gaiman spawned this thread. We probably have 50 books by David Weber in this house (we=my wife and I; my wife is a big fan of that kind of sf) but he never entered even my head when fame came up.
N. K. Jemisin.
What’s Gaiman’s most famous sci-fi offering? I’m legitimately asking because the cited works in the other thread (Coraline, Stardust, American Gods, Good Omens, Sandman, Lucifer, etc) are all some flavor of fantasy. Which would seem to put Gaiman in the same “writes sci-fi but not really a sci-fi author” bundle as King, Koontz, etc.
Collins, from what I’ve seen of The Hunger Games, seems more of a sci-fi author than Gaiman but, again, I’m basing this off the works we’re supposed to recognize him for.
Tolkien surely tops fantasy (which is a subset of sci-fi).
For sci-fi it’d have to be Asimov. Maybe Clarke. I could go with Jules Verne too.
That is not to say there are not other outstanding writers (there are plenty). But the OP wanted the most famous.
He wanted living as well though.
The only living “real” sci-fi author who’d spring to mind for me is Orson Scott Card.
Ack…I missed the living part. 
I like Card’s books but I really dislike him as a person. So much so it makes me not want to like his books. (there is a discussion about this sort of thing elsewhere on this board)
I am not sure there are any living sci-fi writers that are household names (excluding fantasy). I love Simmons and Hamilton but I doubt most know of them unless they read their books.
The Stephen King sci-fi novels/stories I can think of right now:
It - Cthulhu Mythos/godlike alien
The Stand - Post-apocalyptic sci-fi
The Long Walk, The Running Man - Near-future fascist dystopia
The Jaunt - “Hard” sci-fi with FTL travel
The Tommyknockers - Ancient crashed UFO
Home Delivery - Zombie apocalypse via alien satellite
The Dark Tower series - post-apocalyptic sci-fi with fantasy elements
Dreamcatcher - Non-sapient alien invasion
Firestarter, The Institute - Government experiments to create superhumans
I Am the Doorway - Astronaut comes back wrong
Cell - Zombie apocalypse via cell phones
11/22/63 - Time travel
And there’s more I’m sure I’m not thinking of, as I’ve barely touched his post-Dark Tower work and I haven’t read all of his short story collections.
None of these writers are as “famous” as they deserve, I’ll throw out Daniel Abraham and Ty Franck for The Expanse series, Cixin Liu for The Three Body Problem, and John Scalzi.
Add Dan Simmons for Hyperion series.
I really need to get around to reading that!
I think THE DEAD ZONE counts.
For Steven King, you’re also missing Under the Dome, which was subject to a very successful (for the first season or so anyway) TV series in the relatively recent past.
At a guess, maybe James Corey, author of the books on which The Expanse is based? Weir, for The Martian as mentioned previously would also be a good candidate, but I’d bet neither of them have the name recognition of King.
IF (big if) we include fantasy, then we add a lot more candidates, but it also reinforces King’s recognition, as a lot of the previously mentioned franchises are (IMHO) more dark fantasy than sci-fi.
And of course, we still lack a tight definition of what @Exapno_Mapcase wants to consider “Scifi” - because as the poll showed, many of us know Stan Lee, and if you include Super hero works, well, he’d be riiiight up there even if the bubble has largely left us (and that’s leaving out the disagreement’s on what he can claim to have actually “written”).
James S.A. Corey is two people writing under a pen name, so I’m not sure “he” counts.
That one occurred to me, but I figured it fell more under “supernatural” than “sci-fi”.
I haven’t read or seen Under the Dome.
For many, many years, the “Big Three” science fiction writers were Asimov, Heinlein, and Clarke.
All broke out of the insular world of sf and well-known to the general public.
I’m astounded nobody has mentioned Heinlein yet in this thread.
Let me try to justify my pick.
She won the Hugo for best novel three times in a row, and each book was part of a trilogy (the Broken Earth series). BTW, she was the first African-American to win a Hugo Award in that category. In 2021 she was one of Time Magazine’s picks for “100 Most Influential People.”
Unless you know something we don’t, Heinlein is dead.
As are the other 2.
I assumed he had achieved immortality and had fucked off to the moons of Saturn to have sex with his mom or something.
She is absolutely a big name in science fiction literature. I’m not at all sure that, despite the Time Magazine list, she’s really broken through into wider cultural consciousness at this point.
That’s what I get for skimming the thread. Nothing about only living authors in the OP, just “[b]onus points if they are still writing.”
But I now see that the OP clarified further down the thread to specify living authors only.
So…I’m going to vote for Larry Niven. Lots of people are familiar with Ringworld.