I would say MW2, MWII and MW3
Blops where nicely designed games but always ran like shit for me. The connections were always awful for some reason. MW2 the first I played and my son was the one that wanted me to get it.
I think he was 13 at the time. I remember telling him, what ever you do don't let your mom see that no russian mission or you'll never get to play a video game again
Marathon, Lightweight, Commando with a magnum and tac Knife. Pure gold
My best streak ever was doing that on Quarry with the predator, harrier , pavelow