I don't think Modern Warfare was a bad Call of Duty but it's one that was riddled with some surprise old flaws and generally poor game design I found. Infinity Ward's initial bad relationship with the community really soured the long-term experience for some big name players and the format pretty much killed off any eSport aspirations - not that I give a fuck about that like most people but it did leave a bit of a hole.
My two biggest gripes with the game was the amount of needless bugs that persisted and how the file architecture was designed where we would get small playlist and bug fixes yet the update would take around 8-12gb because we had to download and overwrite files we already had. For a game that was over 100gb to start with, I must have had about 200gb of updates on top and I quit playing before Christmas and when Warzone dropped. Why wouldn't they just have internal server side updates instead is beyond me and leave players with just the core files. Treyarch games don't need 50gb of updates a month but IW are bloody amateur developers at times. I uninstalled the game when I had to complete 57gb update and that's not including the reinstallation of shaders.
I still think the last good Call of Duty was probably Black Ops 2. MW is definitely better than BO4, WW2, IW and every other iteration we've had on this gen but it's still nowhere near the quality we should be getting from such an experienced franchise.