For me, it's definitely the story. Even though a lot of people aren't fans of where the plot has gone, I’ve personally been pretty satisfied with it.
The way they’ve built the world out after the initial campaigns has given me so much to overanalyze and theorize about. I honestly haven’t developed headcanons for a game series like this in a long time. My only real gripe is that it was slapped into the Black Ops and Zombies' continuity, which feels pretty odd to me as it is being done through Warzone which is being utilized less as a bridge to interconnect both series' and more of a nostalgia hub and nothing major has been done to highlight this interconnectivity outside of Warzone.
They honestly should had let Warzone stay exclusive to the MW brand if I am honest with you.