what are some single-player games with alot of replayability?

Aside from Fallout/Skyrim which you already mentioned, and Witcher 3/Red Dead Redemption 2/Dragon Age Origins and Inquisition which were mentioned above, the Mass Effect series were pretty sweet. GTAs not too bad too (San Andreas, IV and V mostly. Vice City was nice too but too short.) and maybe something like Grim Dawn or Borderlands 2/3.

EDIT: I forgot Nier: Automata!