Spoilers will be below.
For me, it's gotta be the Water Nation. Not only does it have the North and South Poles, but the Water Nation also seems to have several settlements scattered throughout the world such as the Swamp People. As far as cultural diversity goes it also seems to be the most complex especially since they have settlements that are located so far apart from each other.
So what about you? What is your favorite elemental Nation?
Tomorrow: Favorite Animal