Asimov wrote about the three novas of science fiction in his introduction to The Best of Stanley G. Weinbaum (1974). Three authors who appeared on the scene and instantly changed it forever. There will probably never be another one (any more than there will be an editor as important as Gernsback or Campbell, or an author as important as Heinlein; the field is too mature and too large at this point for one person to be that influential).
The first was Edward E. “Doc” Smith in 1928 with The Skylark of Space. He brought us interstellar travel on a grand scale. It was his first published fiction.
The second was Stanley G. Weinbaum. It’s particularly “A Martian Odyssey” (from 1934), and the character Tweel, that he’s remembered for. He brought us a much superior class of alien. So far as I can tell it was his second published fiction.
And we can now read both these seminal works (and others by the authors) by following the links. All hail Project Gutenberg!
The third was Robert A. Heinlein, who was also the Dean of Science Fiction for a long time (well, one of the four Deans, anyway). His first published story was “Lifeline” in 1939, and in 1941 he was guest of honor at the World Science Fiction Convention (admittedly early enough in the life of that institution that the current thought that 20 years of solid accomplishment was required to even be considered hadn’t taken hold yet). Smith had been the guest of honor the year before.
Gutenberg is of the opinion that none of Heinlein’s works have gone out of copyright yet (which means he was better at renewing the copyrights than most authors; it also helps that he lived past 1978).
Weinbaum died tragically young, of cancer, and isn’t nearly as well remembered as the other two.