I saw this on another forum for a game that has defined this generation, but seeing how this generation isn't done, its a dumb contest, but got me thinking, what game did you feel defined the prior generation?
Off the top of my head, the 3 biggest ones,

The best selling game of the last generation. The game that put open world/sandbox games on the map.

I may not be its biggest supporter, but it redefined what people expected from a FPS on console, and helped put Xbox on the map.
This may be a tougher sell, and I could be completely wrong here, but its the game that showed First Person wasn't limited to shooters, and it was the first game that rewarded exploration with optional story bits (this part confused about, but stuff like BioShock, Arkham Asylum and Borderlands all featured logs, audio tapes, interviews that you don't need to get to complete the game but add to the atmosphere)
So what's your pick?