Well since I've literally done this before, I will go with old school Runescape. In between Pking, Skilling, and Quest, 6 months would be no problem.
Command Modern Air and Naval Operations (Or its brand new successor, Command Modern Operations.
Detail heavy military sim.
Probably destiny 2, it's got tons of content, a rewarding and addictive game play loop, and their always is something else to go get.
I have done that with the original S.O.C.O.M. Game. That was one of the original first person shooter, before the call of duty games came out. It was more realistic, then when call of duty came out it kinda ruined the genre for me. Although I played that game constantly for many months, and it was one of my all time favorites I wouldn’t say it was my favorite. My favorite is usually a toss up between the King’s Quest games, Quest for Glory games, Chrono Trigger, and Dragon Warrior.