Modern Warfare 2

Simply because they had this mode called Spec-Ops where you and a buddy ran actual missions together.

There were 3-4 mission per tier. And there were like 10 tiers. Getting progressively harder as you go.

It had a star system based on what difficulty you beat a mission on. Easy got you 1 star. Hardcore got you 3 or 4.

My friend from work and I spent 3 months playing that mode getting all the stars on each mission. It was probably the most fun stretch I’ve ever had gaming.