Hal Clement (Harry C. Stubbs)
A quiet and underrated master of "hard science" fiction who, among other things, foresaw integrated circuits back in the 1940s.
Minor tweaks to my original answers produced:
For years, this unique creator of absurd and haunting tales denied that he had anything to do with science fiction.
The chief inspirer of the "cyberpunk" wave of the 1980s, his razzle-dazzle futuristic intrigues were, for a while, the most imitated work in science fiction.
A master literary stylist who is also a working scientist.
Arthur C. Clarke
Well known for nonfiction science writing and for early promotion of the effort toward space travel, his fiction was often grand and visionary.
This charismatic cult leader used science fiction as her most effective recruiting tool for new converts.