Just to add another couple..
Many sterling reccomendations already listed above; I'd particularly pick up Neal Stephenson starting right from Snow Crash, he's palpably grown as an author over his career. The last few have just been mind-blowing - Cryptonomicon + System of the World trilogy and the stand-alone Anathem. It's fiction you put down feeling that you've actually learnt something!
From the Golden Age, I'd also take a look at EE 'Doc' Smith - The Lensman series and Family D'Alembert series. The Grand-daddy of space-opera.
David Gerrold's War Against the Chtorr series. "Not quite finished yet". Dense investigation of Eco-Alien Invasion. IMO Falling Skies would have been a lot better-off going this route rather than the way it currently is. Although it would have needed to be HBO then...
Echo David Brin's Uplift War two trilogies. Moving, insightful and hopeful. Startide Rising is the best of the first 3 and I'm sure is everyone's favourite ;)
Well - there's about 10K pages to get your teeth into... :)