My top three according to steam are:
- Stellaris with 1027 hours
- Rimworld with 716 hours
- Hunt: Showdown with 525 Hours
Although the game I've played the most over all my years and outside of steam is definitely Deus Ex. I've put a lot of time into it exploring the single player, playing the multiplayer, and messing around with mods since it came out in 2001. The same would go for Unreal Tournament 2004 which I easily had over a thousand hours in back in high school, before steam was a thing.
Oh man that's a good call, I've got Slay the Spire and Stardew Valley on PC, Android, and Switch and have played a lot on all three platforms.
Have you checked out Downfall? Massive user made mod that's pretty much a second game.
https://store.steampowered.com/app/1865780/Downfall__A_Slay_the_Spire_Fan_Expansion/