My favorite Call of Duty would have to be Call of Duty Black Ops 2 because it implemented competitive rules and game modes. League Play was my all-time favorite in that game.
COD 4 was the GOAT. It revolutionized first person shooters and no other COD has managed what it has. The maps were laid out well and each allowed for good gameplay. Since then each COD has felt like same shit, different game.