Garry's Mod isn't entirely a sandbox game. It has multiplayer gamemodes such as 'Trouble in Terroist Town', 'DarkRP', 'Deathrun' and many more. It has competitive gamemodes to the game as well.
Honestly, I lost interest in the CoD franchise when Ghost's hit the store. I quite liked Black Ops II. Honestly, the best war game i've played is either Arma II or Battlefield. I feel they are quite realistic.