Johto, it seems much more natural to me.
Kanto was only bad looking because it was nintendo's first try at a game like this.
The cool-o-meter for me is this.
Johto-->Sinnoh-->Orre-->Kanto-->Fiore
johto was all natural and mountainous.
Sinnoh was really big and spacy, which made the routes more fun.
Orre was just awesome, the cities and underground was amazing.
Kanto, eh, it was nintendo's first try, so i don't blame them.
Fiore, i just hated it.It was really big, but i don't see the need to make four parts of a city, but make the beginning place hardly littleroot size.