I am a fan of the more cerebral, less action-oriented science fiction. If we separate out fantasy as a separate genre, where Tolkien can be Lord High Master, the King of the Mountain in cerebral sci-fi for the half century of my lifetime has been Isaac Asimov.
A novelist, even his short stories are brilliant. His humor is generally quite humorous. His mysteries intrigue and confuse. His novels, wherever they fall on the science fiction/fantasy continuum, are fulfilling and fascinating.
And when he wrote his epic Foundation Trilogy, not content with simply creating an epic, he created an entire universe.