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.
1056h in Team Fortress 2, which is the highest I can verify at least. I'm sure I've put more time in Sims 4 over the decade but with multiple installs of pirated copies I have no way to verify that lol
I'm surprised I had to scroll down this far to see someone else mention TF2. I clock in at around 2019 hours, although probably around 500-600 of those are from idling (remember idling?!)
Last time I played was in 2019, and last time I played before that was also a couple of years. But for a while, Team Fortress 2 was my life. I eventually got into surfing, and poured SO many hours into that across TF2 and later CS:S.
If anyone who used to frequent Omega Arcade's surf server on TF2 ends up reading this, Mr. Asplundh says hi! ☺️