I firmly believe it's the best game ever created. The sum of its parts--the music, the unique graphical style, the simple but well-crafted story--is of such high quality that it's the videogame equivalent to "Citizen Kane."
Part of the virtue of this game is that it fosters a spirit of adventure, of childlike curiosity and exploration, but also plunges the gamer into an environment of dark desperation later on. This is pivotal. Its characters grow up with the gamer, and it reached me, at least, on a personal level as a result. Not only does it bask in playful minigames, but explores genocide and dictatorship.
The point is, much like the movie Star Wars, it has something for everyone, and its deceptively simple plot masks an accessible and powerful story and game. It's as close to perfect as a game will ever get.