All hail the ruler of Equestria and overall Best Princess - Celestia! ^_  
Japan has always been associated with the Sun, so it seems like a perfect match for her.
 
 
Hokkaido - It’s known as the snowiest part of Japan which fits Princess Celestia’s snow-white coat. https://www.deviantart.com/dashiesparkle/art/Vector-403-Princess-Celestia-3-592806916
 
Honshu - The Yamato Dynasty is the oldest continuous monarchy in the world which fits Princess Celestia’s 1000+ year-old rule. https://www.deviantart.com/90sigma/art/Twilight-Sparkle-Aids-Princess-Celestia-334545482
 
Kyushu - It’s the sunniest major island of Japan, so it speaks for itself. https://www.deviantart.com/vector-brony/art/Princess-Celestia-and-Principal-Celestia-406790070
 
Shikoku - It’s home to the largest dance festival in Japan - Awa Odori. Princess Celestia certainly enjoys dancing judging by all the balls taking place in Canterlot. https://www.deviantart.com/mycaro/art/Flying-Celestia-848295697
 
 
The map is from https://www.pinclipart.com/pindetail/TJThxT_svg-clip-art-wiki-japan-map-vector-png/
 
Princess Celestia belongs to Hasbro