I discovered today that my most played game on Steam is Astronner with about 500 hours, and my second most played is Surviving Mars with about 500 hours.
I played A LOT of World of Warcraft in college and after graduation 15 years ago, I'm a little ashamed to say I put about 2,450 hours into it, but havent played in 5 years. The next is Witcher 3 with 270 hours, No Man's Sky 240 hours and Borderlands 2 with 225 hours.
Definitely WoW too. I mainly played vanilla to Cata and then just a couple of months here and there, especially at new exp launches. Got over 12.000 hours so far.