I can‘t really pinpoint one game, it‘s easier for me to list a top 5ish in no particular order (but even then I‘ll probably answer differently in a month from now when I remember games I‘m forgetting about right now). It goes something like this:
Crusader Kings III
Monster Hunter World/Wilds (the latter if it didn‘t run like shit)
Nioh 2
Rocket League (haven‘t played it in years though)
HITMAN
maybe League of Legends if it still was season 7 lol (haven‘t played that in a while either)
Counterstrike 1.6
Dark Souls II (my first soulslike that I played on release with an active online population, so I have the fondest memories of this one, DS1 and its awful PC port was nothing to make fond memories with for me)
Seems like I‘m a competitive/challenging game enjoyer, but not the „1v1 no-hit bosses“-kind, I enjoy all of these games most in coop if they support it. Overcoming challenges together is my thing.