The Legend of Zelda: Ocarina of Time
No piece of software has affected my life as much as Ocarina has. That's not hyperbole, either: my dwindling passion for games was fanned into an inferno by this game. I expanded my tastes in games and then other media because of this game. Hell, I met my wife because of this game.
What more is left to say about it, though? People have called it flawless, and that's true. Some people call it the best game of all time, and that's probably true too. Its final battle, regardless of difficulty, is seared into the brainpans of everyone who ever beat the game. There is an entire generation of gamers who can whistle at least a few ocarina tunes from memory even if they haven't played the game in over ten years. What more is there to say?
This is one of my favorite stories to tell about Ocarina of Time, related to its design, and I'm sorry if you've heard me say it before but it bears repeating.
In the first dungeon, in the first room, you are required to go up to the top floor and jump off, breaking through an enormous spiderweb in the center of the bottom floor. This isn't intuitive for some people, even though the webbing moves under you when you run over it, hinting at its elasticity and the limits thereof.
At the top floor there is a ledge that you can jump off of to hit the spider web and get into the basement, but many players would never think to do this - except that there is a rupee there. Out in the air, just far enough that you definitely have to jump to get it, is a rupee. The first time I played I made a flying leap for that rupee, fell approximately seventeen miles, and overshot the center of the web, stretching it to the limit without breakign it. A lightbulb went off in myhead, I ran back up to the top of the room, and jumped off again. Bull's-eye. That was my real introduction to this game, to this series, that one flawless piece of design that helped me get it when I might not have otherwise.
Ocarina of Time is full of moments like that, nuggets of genius and wonder that should be so intuitive but are so rare in almost any other series. It is in a class of its own.