I'm not the type of person who can enumerate my favorites in a snap, but Ender's Game is definitely, definitely one of my favorite books of all time. I've read it thrice already, I think, and I don't think I'll ever get tired of reading it.
It's the type of story that wrenches at your heart without even trying. And apart from the emotional nd philosophical side of the story, the sci-fi and strategy side is also good. Although I think that side of the story wasn't really highlighted. In fact, I don't think any part of the story is particularly highlighted. The story just presents its reality without shoving any part of itself to your face, which was really effective, in my opinion.
I will definitely remember Ender's Game throughout my lifetime.