Lots of interesting stuff here. I think I've probably read thousands of SciFi books over the years. Many authors have been mentioned already: Neil Stephenson - SnowCrash and Diamond Age, Orson Scott Card - Enders Game (a fantastic classic), Alfred Bester, Frank Herbert - Dorsai Experiment, Niven and Pournell - Legend of Heorot and Mote in Gods Eye, Dan Simmons - Hyperion novels, Anne McCaffrey - Dragonrider books, and John Varley. All great stuff.
Julian May - The Many Colored Land (and connected books): For a vivid description of Earth Pliocene history mixed with SciFi
Vernor Vinge - A fire Upon the Deep: For a the idea of profound consciousness beyond gravity wells
A. A. Attanasio - The Last Legends of Earth: For a romp through reality and time from the subatomic to the cosmic
David Brin - StarTide Rising: For the idea of uplift, the mystery of the progenitors and the most flawed aliens in the cosmos
Of all of the the books I've read, one stands out for me as the best ever. It's by a relatively unknown author named Daniel Keys Moran. The book is called _The Last Dancer_ and it is spectacular; with ideas and prose to satiate the brain and action to thrill the heart