The best PC simulation games

What are the best simulation games on PC? From the all-time greats that are still worth playing; the sims with modding communities who breathe eternal life into them; the facsimiles of an aspect of reality so niche you can barely fathom their existence, we have everything you need here.
Sims differ from other PC games in that their raison d'être isn't necessarily to entertain, at least not primarily. You want fun and excitement? Tough. Operating a submarine is not that. It is, however, fascinating, terrifying, and hugely satisfying as a long-form experience.
Such is the nature of the genre that seeking out the best PC sims is like asking for the 'special' stock the shopkeeper keeps in the back room. They're not all best-sellers, but these are the titles with dedicated communities offering deep tutorials and incredible mods, that let you get as close to hauling a trailer full of plumbing parts across Europe, driving an F1 car, or piloting a military aircraft as national security laws will permit.
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