Call of Duty Black Ops Cold War
I sunk an enormous amount of time into that games zombies mode. I think I have about 30 days of play time (compared to the 8 or so hours I spent in multiplayer).
I grinded out every single aetherium upgrade, ever single piece of Intel on every map, every single zombies challenge (ie make it to round 20 without being hit, get to round 100, etc), and most importantly I have the dark aether camo for every single gun. I would throw on a tv show in the background, load up Firebase Z, and grinding out 2500 headshot kills with every gun, one at a time. Typically that amount was enough to get the gun to max rank, and complete all of the other camo challenges for the gun at the same time. When a new weapon came out, I'd unlock it and repeat the process. Each run, I'd extract at round 30 because any higher would start taking too long, but any lower wouldn't be a substantial enough amount of kills to for gun exp.
I have no idea why I was so autistically obsessed, the game would piss me off so much because every new patch introduced a myriad of new problems that would take weeks to get fixed. Shit like "Pack-a-punched sniper rifles lost all penetration", "crossbow has no aim assist from Deadshot Daiquiri", "nail gun aim assist from DeadShot Daiquiri has you snap 2ft to the left of the enemies head", or my favorite; "map has loading bug, and can no longer be played unless you abuse a pause buffer during a specific segment of the intro cinematic"