Lots of great choices in here.
In a slightly different vein from most suggestions above, I'll preface this by saying open world games are my crack.
RDR2 is one of the best ever single player games if you like that kind of thing. It takes a fairly decent computer to run it. Mine is a 2500k, 16GB RAM, and an R9 380. It juuuuust barely runs it politely, I'd consider it bare minimum spec. If fantasy is more your jam, Witcher 3 is truly excellent too.
If your computer is a bit older, Skyrim doesn't take a real beast to run and there's plenty of fun to be had. Fallout 4 is a bit newer and is the same idea, but post-apocalyptic.
Wiley.