Answer
Isaac asimov.
he wrote extremely
interesting and deep scifi stories, but he also wrote tons of other stuff - popular science, mystery novels, chemistry texts, biblical commentary, and even newspaper columns. that he could write so prolifically, well, and knowledgeably outside of the science fiction genre has always told me that he was the best of the science fiction writers.
other favorites are larry niven and arthur c. clarke.
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edit: for ianhad .. david niven was great, but he was an actor not a scifi writer :-)
http://en.wikipedia.org/wiki/david_niven
