I'd say BO2 is the best yet. Well written story, a good and sympathetic villain, multiple ending, great variety of weapon (futuristic and historical), you name it.
Honestly think this discussion's answer is different for everyone because i think most of our opinion on the best cod is based on the one that we had the best memories with. For me its BO2. Me and the boys would get on every single night and play multiplayer till midnight then switch to zombies till 2am