My fave author in the genre probably remains Asimov, though Bradbury and Clarke are nearly as good.
I've never been a great fan of sci-fi, although I read all the usual Jules Verne and H.G.Wells titles many years ago. I've taught some of the twentieth century classics to students -
Brave New World and
Nineteen Eighty-Four, for example.
From my limited knowledge of the genre, I would agree with TRex's ABC - Asimov, Ray Bradbury (especially) and Clarke and add a D - Philip K. Dick, an H -Frank Herbert's
Dune novels and a W - John Wyndham.
American writer Ira Levin is probably best known for writing novels which became classic films -
Rosemary's Baby,
The Stepford Wives and
The Boys from Brazil for example although he did write one other excellent sci-fi novel which, as far as I know, has never been filmed called
This Perfect Day. Well worth reading if you can find a copy.
Finally, let's not forget that one of the greatest and earliest of sci-fi novels recently celebrated its 200th birthday - Mary Shelley's
Frankenstein.
What an achievement for a young lady barely 20 years old!