Black ops 2 and 3 zombies. I have 700 hours in bo3 and 600 from bo2 on PC. besides that I also played it a couple years on ps3 so it's probably above 2K together.
Otherwise it's dead by daylight with 1500 hours total.
Metal Gear Solid 2 is my most played game being my first mature game and I replayed it throughout my life even to this day. Second most would probably be Csgo with a couple thousand hours thrown in over the years.
The most impressive playtimes I've seen though was a friend of a friend who (probably used to a bot or something to achieve the hours) had hours on all the Nekopara games on steam from the day they were released on steam to this day, till going too.
Last I checked they were on several thousands of hours lolol
Blacks ops 3 pc 2,000 hr. The custom zombies make me come back to it and it supports remote play anywhere so you can stream it to a friend and they can play too