It's not one game but a series.
Fallout.
On PC all but 76 can have quest mods adding hundreds of hours of gameplay to them.
The storyline is lengthy and done well and generally adding levels of "free will" to them. You can ignore the main quest in these and make those games "infinite" in length.
Gritty. Post apoc. PC starts out with nothing or very little and must "make your way in and survive that world".
76 is it's own unique thing.
I've played it solo since launch except for random times years ago.