Fallout 4. A lot of this is going to be due to mods.
Wargame: Red Dragon. Intended to be played multiplayer; I played it single-player. Steel Division II is a far better single-player choice if you don’t mind the different setting, as the AI is much more interesting.
Fallout 76, the only entry here I actually play multiplayer (and even that to a minimal degree; that game tends to have players having pretty minimal interaction with each other unless they’re actually trying to play with each other). I would recommend playing Fallout 4 over Fallout 76 unless you specifically want multiplayer; Fallout 76 is just the closest thing to “more Fallout” short of a Fallout 5.
Not run through Steam, so no Steam stats (though available on Steam) but I’m sure that they’re way up there:
Cataclysm: Dark Days Ahead. Free and open-source, though there’s a commercial build on Steam if you want to effectively donate. If not, can download from their project page.
Dwarf Fortress. Free, though there’s a commercial build on Steam with a fancier, more-approachable UI and such.
Would be interested to know what games you have >500 hours in. Especially if they aren’t multi-player online games.
Oxygen Not Included
Caves of Qud
Fallout 4. A lot of this is going to be due to mods.
Wargame: Red Dragon. Intended to be played multiplayer; I played it single-player. Steel Division II is a far better single-player choice if you don’t mind the different setting, as the AI is much more interesting.
Skyrim. A lot of this is going to be due to mods.
Rimworld
Civilization V
Fallout 76, the only entry here I actually play multiplayer (and even that to a minimal degree; that game tends to have players having pretty minimal interaction with each other unless they’re actually trying to play with each other). I would recommend playing Fallout 4 over Fallout 76 unless you specifically want multiplayer; Fallout 76 is just the closest thing to “more Fallout” short of a Fallout 5.
Not run through Steam, so no Steam stats (though available on Steam) but I’m sure that they’re way up there:
Cataclysm: Dark Days Ahead. Free and open-source, though there’s a commercial build on Steam if you want to effectively donate. If not, can download from their project page.
Dwarf Fortress. Free, though there’s a commercial build on Steam with a fancier, more-approachable UI and such.
Dungeon Crawl: Stone Soup, though that’s going back a few years. Free and open-source.
Some others with a fair bit of playtime:
Steel Division II. Primarily a multiplayer game, but I only play single-player.
X3: Reunion
Mount & Blade: Warband. The Prophesy of Pendor mod adds a fair bit of time to this.
Elite: Dangerous. Though I don’t remember how I accumulated that many hours. Wasn’t super-impressed with the game. Probably pretty in VR, though.
Stellaris
Noita
Kenshi
Nova Drift
Starfield
The Sims 3
Starbound
Rule the Waves 3
Wasteland II
Fallout: New Vegas
Carrier Command 2. Primarily intended to be played multiplayer, but I play single-player.
Kerbal Space Program
Total War: Warhammer II