I think GTA & Elder Scrolls should be separate because they are 2 different types of games. How do you compare a city with a fantasy world?Yes, and by quite a large margin I'd imagine.
SA is in GOAT talks. IV was good, but I think most GTA players agree that it was severely lacking in the game play department, in terms of variety. Lets also not forgot just how shitty the driving was in IV either.
Anyways, I think a distinction should be made between "Open World Sandbox", and "Open World RPG". I think both GTA and ES are the best in those respective categories.