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.
On steam it's dota 2 with about 3k hours but really my most played game is the old f2p Combat Arms. I nearly exclusively played that game after school to bed for like 4 years, I probably had over 6000 hours logged. Still talk with the friends I made there a decade later.