7 days to die, surprisingly. I've played plenty of the game, but didn't realize I have almost double the next game's playtime on it compared to everything else. Funny thing is I take months long breaks from it frequently.
The games i have invested the most hours was Final fantasy tactics 90 hours 2 walkthroughs, Elden ring 101 hours, Helldivers 2 96 hours, Baldur's gate 3 80 hours.
WoW would probably take the prize over the years but I have no idea of how many hours I spent on that. On Steam Caves of Cud takes the cake with 1500 hours.
I have no idea how long I spent on League of Legends... I played it when I was younger and that was... over a decade ago. It's probably 10k+ or so at this point. It's split between many different accounts. Second would be WoW, with many hours as well.
games with modding usually keep me hooked for quite a while so the likes off: minecraft, skyrim(889 hrs on steam and many hours on console), fallout 4(only 98 hrs as i used to play it more on console), stardew (466 hrs) and terraria (800 hrs) did sink a lot off hours in some multiplayer games as well like payday 2(538 hrs), eso (290 hrs) and warframe (175 hrs but played it more on console)