I think my highest is still Kerbal Space Program by a country mile, and that's only what Steam itself counted and doesn't count the at least similar number of hours I likely spent in pre-release versions back when I was first downloading it directly from the devs with no Steam version in sight. So the real numbers are probably somewhere in the 4 to 5 thousand hours range. Dwarf Fortress is probably pretty far up there too but like KSP those hours mostly aren't recorded anywhere. Factorio, Avorion, Starbound, and Terraria are also shockingly high, into the thousands, and a bunch of others like Stardew Valley, Sims 3, X3TC, X4, Empyrion, and Stellaris are knocking on the door of the thousands too. But nothing comes close to KSP for me.

There's also a few I'm surprised are not higher up: Subnautica, Satisfactory, Oxygen Not Included, The Long Dark, ARK Survival Evolved, Rimworld, Astroneer, Slay The Spire are all games that I feel like I've played an absolute ton of, which spend a lot of time living rent-free in my head, but the numbers don't actually show that I've played them that much, low hundreds of hours at most, some only in the high tens. No reason to believe that Steam's numbers aren't accurate here, so I can only conclude that those games are somehow denser and provide similar gameplay value in a shorter play session with less time investment, because yeah my brain has devoted a lot of pathways to some of those.