Deserted island and only 1 game? Probably skyrim. There is just so much to do in that game and I think it has the longest, single player re-playability. You could go around and do every single little side quest which there are a ton of. Or just go exploring since the world is freakin huge. But if we had wifi to go multiplayer I'd probably use the internet to get help lol. But assuming you couldn't use the wifi to get help I'd probably have to say GTA V, and probably for the same reason as skyrim, You can play a gta game forever without actually doing any missions.