What games have defined this generation for you and why?

[Famine]

R:FoM - The multiplayer component is what makes it stick out. 40 player online for a console game was, still is, pretty huge, and now we can expect 60, and an 8 player co-op.

Resistance 1 defined what a FPS should have for online console gaming, Resistance 2 will re-define it.