Gen 4
This is a very unpopular opinion, especially here, but I very much like how Mt.Coronet divides the region. This mountain that just seems so simple ends up being so vital to the game, and theres always more you learn about it as you have to backtrack to it. At first it just seems like a simple cave, then we see fog, and then it turns out to be a full scaled mountain that you can explore the outside of too.
And the music is just so good
https://www.youtube.com/watch?v=gWkeXyCnEQI 
