In terms of pure aesthetics, Ballonlea looks spectacular. However, the actual layout is really boring. Glimwood Tangle is also cool looking (I like the mushroom lighting gimmick) and overall has quite a few areas to explore, so I'll probably go with that.
As you can probably tell, I love the fairy themed stuff. There's a reason Laverre was one of my favorite, if not singular favorite, cities in X and Y...