Fallout 2 and Planescape Torment are tied in 1st place for me. They are the only RPGs that every time I replay them, they feel like a blank canvas, like they always have a different story to tell if I play them differently. A "sandbox" doesn't need a megahuge world full of uncanny robots to be diverse, and its story does not have to be a piece of shit. These are the lessons that those 2 games tried to teach, but they seem to have been forgotten over the years.