There are to many to choose from.
H.G Wells has to be one of the greatest as his stories have survived the test of time and are still enjoyed today.
Arthur C. Clark will always be remembered as one of the greats of the genera during the 20th Century.
Isaac Asimov wrote some of the best Robot related Sci-Fi stories
Philip K. Dick wrote some of the best stories to have been turned into Movies.
Harry Harrison for me has to be one of the greats. I was introduced to his work about 14 years ago while I was at college. I've never looked back.