Oh, this is too easy! Sinnoh! Hands down. Not only is the region is absolutely beautiful, the music is splendid, and all of the cities and towns have such warm, homey feels to them. Jubilife is lively and bursting with life, Twinleaf town is serene and quiet, whereas Snowpoint city is a stunning winter wonderland, the region is full of diverse places. The pokemon in Sinnoh are also my favourites, which helps.
If I were to live in one particular place in Sinnoh, it would either be Hearthome city, or Florama town. I love flowers and Florama town has such a lovely, tranquil design and I would love living in a town always in full bloom! Hearthome city, on the other hand has a cheerful, cozy atmosphere. It's the perfect city to live in when you love Pokemon contests. :)