Pyrous Mountain - is my favourite because of great soundtrack, it's well designed in terms of visual experience even if i got lost once (i don't know how i did it though).
Byxbysion Wasteland - it's my least favourite because of overall mess. It's hard to figure out where you need to go or what to do in the begining. I had hard time walking throught that place and memorize several parts of the dungeon. I understand how wasteland should look like, but that "put all random objects you saw to far, mix them into total mess and tell people that new poison gym is in another side of this" isn't very possitive first look on topic.