For me its very difficult to pin down just one.

Every generation had something going that can be considered great. And there are many benchmark games like, say, Mario Brothers, Zelda, Tron, Zaxxon, Centipede... that's just off the top of my head and the list goes on and on.... so with that in mind, these are some of mine:

Old School:
Space Invaders (atari 2600)
Miner2049'er (colecovision)

90's PC Online Multiplayer :
Quake (1,2,3)
The Quake3 Multiplayer set the stage for every mp I've played ever since.

Currently (read last 5 years):
Borderlands (2009)
Battlefield Bad Company 2 (2010)
Dark Souls  (2011)
Journey (2012)
The Last of Us (2013)
[The jury is still out for 2014]