Call of Duty: Black Ops 3\n\n Black Ops 3 is arguably the longest Call of Duty game. Most people report finishing the main story in about nine hours, which is certainly a respectable amount of time for a first-person shooter that releases just about annually.
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.