

-
Fallout 4. A lot of this is going to be due to mods.
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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.
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Skyrim. A lot of this is going to be due to mods.
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Rimworld. A lot of this is going to be due to mods.
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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).
Not run through Steam, so no Steam stats (though available on Steam) but I’m sure that they’re way up there:
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Dungeon Crawl: Stone Soup, though that’s going back a few years.
Some others with a fair bit of playtime:
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Steel Division II. Primarily a multiplayer game, but I only play single-player.
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Mount & Blade: Warband. The Prophesy of Pendor mod adds a fair bit of time to this.
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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.
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Carrier Command 2. Primarily intended to be played multiplayer, but I play single-player.
My electricity costs to run the game are higher than the cost of the game itself at that point.