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.
Path of Exile, around 3k hours and it is still my favorite. Every 3 months you get to play again with a different twist, and Path of Exile 2 (just an update, but a huge one) is being worked on and will likely release next year.