Don't have one. I'm not actually sure there's any sci-fi writer I've read more than one book by, unless you count Douglas Adams.

I'm still inclined to say Tolkien for pure fantasy. Nobody else is immediately jumping to mind for speculative fiction as a whole, either...

I'll say A.S. Byatt. There are definitely other candidates, but I liked all of her books that I've read (which is most of them), and loved a sufficient number to put her in that spot, I think. Of course I might name somebody else tomorrow, but Byatt would still be in the top three or so.

As to why, I'm not sure. I guess I really like her standard combinations of themes (romantic and family relationships, art & literature, social changes and evolutions, religion appear in essentially all of her works), and she's a very solid and reliable writer for the most part.