Call of Duty: World at War is considered the hardest game in the series, with intense enemy presence, constant fire, and challenging missions that make it the ultimate experience in terms of difficulty.

1. Black Ops II. Call of Duty: Black Ops II took everything that Black Ops did right with the Zombies mode and perfected it even more. No one thought this would have even been possible, but Black Ops II created the perfect and best Call of Duty Zombies mode of all time.