Everyone always recommends Ender's Game, which is an excellent book, but there does exist other excellent stuff out there and I get a little tired of seeing the same stuff over and over and over. Depending on your taste:
Jack L Chalker: Well World series (there are three separate series), Soul Rider series, Changewinds, and many others.
Peter F Hamilton: Night's Dawn trilogy (The Reality Dysfunction, The Neutronium Alchemist, The Naked God), he's got another series out there as well
Joan D Vinge: Snow Queen, Summer Queen
Daniel Keyes Moran: Continuing Time series (Emerald Eyes, The Long Run, The Last Dancer)
James H Schmitz: Witches of Karres, Federation of the Hub series (Telzey/Trigger stories)
Iain Banks: Culture series (Excession, The Player of Games, and others)
John Varley: Steel Beach, Titan, Wizard, Demon