Eryth Sea is the best looking area in the game but it's a pain to swim around it and there isn't much interesting going on in the vast stretches of water. I think Gaur Plains is the perfect combination of looks and level design. It has a lot of different elements to the terrain. You got the big open fields, the bridge overlooking the lake and those strange mushroom platforms, those arching platforms over the tortas, forest, Sky Stage, Windy Cave, Spiral Valley, and some tall cliffs to overlook everything. Plus it has arguably the best view of the Mechonis.
thats why you use the teleports and only swim short distances when you have to!