Let Them Eat Cake, and while I originally felt like it replaced my top favorite, I realized they are such very different books that they can't be compared.
The book is Ender's Game by Orson Scott Card. When I first read this book, it seriously blew me away. I loved it so much. This is one of those books that I recommend to everyone. I don't care if you like to read or not. I even made my husband (who is definitely NOT a reader) read it, and he liked it too. I don't care if you're not a science fiction fan--this book isn't too heavy on the science, and it's very character driven.
Here's a synopsis:
Aliens have attacked Earth twice and almost destroyed the human species. To make sure humans win the next encounter, the world government has taken to breeding military geniuses -- and then training them in the arts of war... The early training, not surprisingly, takes the form of 'games'... Ender Wiggin is a genius among geniuses; he wins all the games... He is smart enough to know that time is running out. But is he smart enough to save the planet?