I personally think a 'defining game' should have influenced other games. At least that's how I understood the thread.
Oh, if we can include PC:
It re-invented a genre and popularized it. WoW influences can be felt in many genres (for example story/quest driven gameplay, social aspects).
I'm trying to think of another defining DS game.
But first, how could we forget Guitar Hero??
It spawned not only 5 sequels or so, but 1 other series and several competing games.
Ah, got it.
Spawned a load of puzzle games and pulled in a huge user base.