Minecraft easily. The game doesn’t record hours but if I were to guess it would be at least 10k hours.
Second most played game has to be Diablo 2. In Diablo 3; I have 350 hours on one character, 200 hours on another, 125 on another and various temp character that I play during seasons and delete probably another 400 hours there. With all that being said about Diablo 3. I’ve definitely put more hours into Diablo 2. I would guess at least 3000 hours for Diablo 2
Game with the most hours played on Steam is Town of Salem at 750 hours and then Grim Dawn at 360 hours.