Wolfenstein 3D!
Doom, when it first came out, was completely awesome.
Then came Quake and the original Team Fortress mod, back in the days of the dial up modems and AOL. That mod was so much fun, nothing like today's Team Fortress 2.
I've played all the Call of Duty's but the one I still play today is the COD1 Expansion Pack: United Offensive. Can't beat the multiplayer. It has a smoothness and a precise crispness that just isn't found in the more recent CODs.