The first Doom, back when you used your phone so your computer could call your friends computer.
Half-Life series
Unreal Tournament 2004
Counter-strike: Source First time I ever joined a clan was because of this game and I still have friends from around the world because of it.
BF3 I started this series with Bad Company 2 and hated it with a passion, a friend talked me into the beta for BF3 and it is still my favorite in the series, especially the Close Quarters maps.