Multiplayer : Metal Gear Online. Played it from 2008 until it was shutdown back in 2012. Best gaming community ever. Always got GGs after every match and what bitching did occur didn't last long as both people usually got voted out of the server.
Singleplayer: Call of Duty 4. It was also my second most played Multiplayer game. Call of Duty 4 is truly better than every other Call of Duty.