Of all the Call of Duty games released on Steam so far,
Black Ops III has been the one with the most longevity.
And it's not very close. Here's the top 10 most currently played Call of Duty games on Steam:
Full list here.
Black Ops III has been selling quite well every time it's on sale, even sometimes being inside the top 10 global sellers like it currently is, which is impressive for an almost 7-year-old game. The game also gets a nice boost in concurrent players every sale, as displayed by
Steam Charts:
I'm sure you can pinpoint a lot of the game's success to its beloved zombie mode, but it's worth noting that Black Ops III is the only Call of Duty game with
Steam Workshop support. I'd like to think that has a lot to do with its longevity, in both sales and playerbase. I'm hoping one day we'll see them embrace the modding scene once again in future titles