Ender's Game - Orson Scott Card
In the future, Earth has been attacked once by the Buggers, an alien race of insect like creatures, and Earth's unified military expects another attack. To prepare for the impending invation, the best and brightest children are recruited into Battle School--a highly advanced military training physicility in space. They are searching for the one child who can save humanity from certain destruction . . . and they think they have found him in Andrew "Ender" Wiggin.

That's Ender's Game. It's considered among the most influential books in histroy; it is the standard against which all other science fiction books are compared. It is the winner of the two most prestigious awards in science fiction, is #59 on the reader's list of Modern Library 100 Best Novels, and is on the American Library Association's "100 Best Books for Teens. It is even on the U.S. Marine Corps list of recommended reading for officers in training. (Thank you Wikipedia)

Not to mention that Ender's Game contains the greatest surprise ending in the history of surprise endings. Ender's Game.

For me, 4 stars out of 5 because the ending (after the surprise) is kind of weird. I can't share anything about it without spoiling, so I'll just say I didn't feel perfectly satisfied at the end. But I'll still recommend it, Ender's Game really is a fantastic story.

Plus, I strongly recommend that once you've finished reading Ender's Game, you jump right over to Ender's Shadow. Instead of a sequal, it retells the same story, only from Bean's point of view instead of Ender's. It may sound gimicky, but it works to great effect. For me, Ender's Shadow is a 5 star book.