Ruth. Because he was a great pitcher and the best hitter, and saved the game of Baseball.
But I've always believed that his impact on the game was somewhat exaggerated because he was a dominant player at a time when the game was changing dramatically. So he set all kinds of records that were considered outlandish and superhuman at the time ... and yet most of them have been broken. That last item is what really sets Gretzky apart from anyone else.