if you want to factor in sheer replayability and possibilities, then objectively it may have to be Minecraft or Dwarf Fortress.
while i don't like either of those games and my favorite is Prototype i honestly think those may have the most potential of any singleplayer game out there
while i don't like either of those games and my favorite is Prototype i honestly think those may have the most potential of any singleplayer game out there