Do you mean the original MW or the rehashed one? The last one I probably played was MW2. My friend (not the one mentioned above, another friend) loved playing the co-op wave style missions, and the stealth missions.
I remember playing the one that had Christopher Meloni in it and it ran like shit on my PC which was high end. After that, I lost interest. When I moved in with the one that was addicted, I’d play with him just as a “bonding experience”, then he wanted me to play online with him , which I did occasionally. I remember trying to do the “secrets” and then having some asshole bitch me out because I moved an inch off of a platform or didn’t kill someone in time. Fuck that.