For Fantasy, I would recommend Guy Gavriel Kay. His 1990 novel named Tigana is brilliant and a good introduction to his work. (He also assisted Christopher Tolkien bring The Silmarillion to fruition)
For Sci-Fi, I'll throw out an author by the name of Verner Vinge. His recent novels include A Fire Upon The Deep and the sequel/prequel, A Deepness in The Sky. I picked them up myself thanks to a recommendation from Id's John Carmack, who praised them in an article I read.