I'm going to go with the Mysterious Beyond. Sure, the Great Valley would be a great place to live (hence its name) but think about it this way - the Mysterious Beyond is, technically, anywhere that isn't within the Great Valley, yes? Well, the Great Valley is just one valley. In the Mysterious Beyond, you can find anything. It is, after all, mysterious. You've got places like the Land of Mists, the Big Water, the mysterious island (yes I know that's another option!) the crater from the tenth movie and all sorts of other places...