For those of you old enough to remember the combination of war and childhood, Ender's Game was a classic. I just remember thinking of how cool it would be to be able to play around in the war rooms without gravity.
Card didn't stop, and he combines deep character development with complex, well thought out plots. My most recent favorite is this Seventh Son series, which covers an alternative American Revolution and follows an extraordinary child through adulthood while bringing a new version of what might have been in the American colonies. I just got the fourth in the series and can't wait.