It's the worst guys, let's face it.
Here's the best games in the series based off of their modes and devs:
Campaign: OG MW2/WaW
Multi: OG MW2
Co Op: Black Ops 3 Zombies/MW2 Spec Ops
Worst
Campaign: WW2
Multi: Black Ops 6
Co-Op: WW2
Best games from the devs (note that these are the main developers of the games):
IW: MW2
Treyarch: Three way tie between WaW, Black Ops, and Black Ops 2.
Sledgehammer: … Just no. Simply no. Don't care about Kevin Spacey or the stories. They made the worst zombies, multi, and campaigns.