What makes the most important game of a decade? Most many units sold? Best critical rating? Most swear words per sentence/nude pixelated women? I think we can take it one step farther. How about most consoles sold?
Individual game don't sell consoles. That's a general rule that you find for game consoles, something PC gamers don't have. If you wanted to play Doom 3, you bought a brand new videocard, and that was final. But if a game comes out on the N64 and you have a PlayStation, screw it, you'll play it at a friend's house.
And then there was Halo.