Most anything by Heinlein, Asimov ( Foundation series is fantastic as well as fun ), Clarke, Frank Herbert ( Dune and many others ), Harry Harrison is a fun read ( Stainless Steel Rat series, Deathworld trilogy ), Niven ( a good read but his endings alway suck. Still worth reading ), Joe Haldeman ( the Forever War, Tools of the Trade ).
Fantasy - Marion Zimmer Bradley ( Darkover series ), David Eddings, Terry Brooks ( Shannara series ), Dave Duncan ( his books are a fun read ), Stephen Donaldson ( Chronicles of Thomas Covenant ), Fred Saberhagen ( Swords series, Lost Swords series ).
Those are some of the books I have within sight as I'm posting.