As in: love or hate JRR Tolkien as you please, but you may not ignore him when it comes to the fantasy genre. Venerate or despise HP Lovecraft for any number of reasons, but when we talk about horror we are ultimately using concepts and conceits that he defined and developed. But if there’s a single figure of the science fiction field that similarly towers over the landscape, I do not know his or her name. Bob Heinlein comes close. So does John Campbell, although that would have been more true thirty years ago.
*cue arguments in the comments and on Twitter*
Moe seems to have skipped right over the obvious candidate right there. I'm a big fan of John W. Campbell, Jr., as the editor of ASF who defined the SF genre and particularly the "hard SF" occasionally sneered at as "engineer porn", but as an author, JWC belongs in the Golden Age of Pulp along with his contemporaries Doc Smith and Edmond Hamilton. Nobody today, or even in the last fifty years, compares themselves to Campbell.
Uncle Bob, though...the man has been dead for almost thirty years, but the mere mention of the author or his works can still set supposed adults to frothing with rage. I have lived long enough to see authors in two separate generations have their best-known works compared to Heinlein, and likewise to see his critics flame out and disappear into obscurity. Whether one likes it or not, anyone writing military SF stories that deal with elite infantry is going to find those stories compared to Heinlein's Starship Troopers. Conservative and libertarian SF writers hold up Heinlein as an icon, a touchstone, an inspiration; leftists, on the other hand, have reviled him for the last sixty years for leaving the Socialist Democrats of Upton Sinclair and becoming a fervent nationalist. They complain that Heinlein's portrayal of women was offensive and denigrating, when it's well known that he modeled a lot of those characters on his wife Virginia, an accomplished woman in her own right as an engineer and quite a babe besides.
There are a lot of other SF writers who had an impact on the field; there are others who were as popular as Heinlein. You seldom see people compared to Ray Bradbury, Arthur Clarke, or Harlan Ellison, though. That's because for better or for worse, the Dean of Science Fiction is also its gold standard, the towering figure against whom all others are measured. I could probably make the argument that his impact on SF has been greater than Lovecraft's on horror and Tolkien's on fantasy, but that's not my point. It is comparable, and that is enough.