Hoo boy. I own Vinge's A Fire Upon the Deep and have tried at least four or five times to get through it (without success) - though The Peace War is a pretty good read. Ditto for The Forever War - I swear he called it that because it seems like it takes forever to read it. (Truthfully, the first one isn't half bad, but avoid the sequels like the plague.)
My recommendations:
Isaac Asimov, the Foundation trilogy (Foundation, Foundation and Empire, Second Foundation).
Frank Herbert, Dune.
Robert Heinlein, Stranger in a Strange Land (Yes, I know I'm duplicating previous suggestions.)
Orson Scott Card, Ender's Game
Larry Niven, Ringworld
Joan Vinge, The Snow Queen
Connie Willis, The Doomsday Book
Robert Silverberg, Tom O'Bedlam or The Book of Skulls