Second Runner Up: Rock Band

It's hard to tell at this early stage where a game will stand ten or twenty years from now, but I'm willing to cast my ballot early for this one.
It seems that after 2 years, the mainstream has finally caught on to Guitar Hero and how great it is. This creates a great opening for Rock Band to come in and take over the multiplayer space that Guitar Hero never can. Rock Band has succeeded in creating a new genre of game.