Ok, I'll give you categories based on reading length:
Series: C. J. Cherryh's Foreigner series. These SciFi books have realistically little action, but the psychology, and the realistic portrayal of alien civilizations meeting and living together is amazing.
Trilogy: C.S. Friedman's The Coldfire Trilogy. This is the one trilogy that I've probably read 10 or more times. It's fantastic. If I had to suggest one thing to read, it'd be this. Plus the original cover art was done by Michael Whelan. How can you say no to that?
Single Book: Aldous Huxley's Brave New World. My All time favorite Sci-Fi book. Ever. It's not an easy read for everyone, I mean, it's old, so you have to get used to the story telling style. Once you do though, it's a fantastic story.