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.
Definitely factorio, but I can't tell you how many hours. I started playing on steam, but then downloaded it from factorio.com and had multiple installs so I could play different mod packs. Steam says 1100 hours, but it's got to be 3 or 4 times that, total. I've played multiple saves that took several hundred hours