There is only one author whose complete works I have thoroughly enjoyed time and time again, and that is Robert A. Heinlein.
I'm not a big science fiction fan - I find much of it derivative and cold - but Heinlein's work (more accurately defined as "speculative fiction" by the man himself) contained at its core a deep understanding of humanity and the human condition, communicated by some of the most interesting, varied and convincing characters to ever come from fiction.
As was often the case for sci-fi in Heinlein's day, the genre was mostly an allegory for modern life that allowed him to make bold social statements without fear of reprisal. This meant that the characters and dialog were always the focus of the story, and not the technology. However, Heinlein's deep passion and understanding of mathematics, physics and science, as well as the military, is very apparent in all his writing and incredibly infectious. It adds an air of credibility and authenticity to his stories.
His earlier novels in particular are charming, often introducing young and naive protagonists who view the galaxy - including the races that populate it and the technology they utilise - with a child-like wonder, through which the reader can identify. Because the themes he presents are universal - crossing from childhood into adulthood, redemption, camaraderie, love, a strong sense of family, the struggle against oppression and good old fashioned exploration and adventure - his work appeals to all kinds of readers. A love of science fiction is certainly not a pre-requisite for enjoying the worlds of Robert A. Heinlein.