I love the Ender books. My all time favorite Sci-Fi author is Robert Heinlein. He has given me thousands of hours of pleasure. Also, he has shaped my attitudes. When I had children, I made sure that they were exposed to his world-view, too. "Back in the day" he had some of the few strong female characters.
if you're looking for a series:
Lord of the rings
game of thrones
the belgrariad by david eddings (great series that got me into fantasy)
Any Ray Bradbury
Cat's Cradle (this counts as sci fi right?)
Ender's game (this is actually part of a series but this is the best)
The last post here was over a year ago, so I hope someone is reading this! Is anyone here reading the Discworld books by Terry Pratchett? I can't get enough of them! They are funny, intelligent and meaningful. Highly entertaining.
I'm also reading Cloud Atlas. I haven't seen the movie yet. I'm looking forward to the release of The Host in the spring. The book is by Stephanie Meyer but it could not be any less like Twilight!
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.