It's a threeway tie between Slateport, Fortree, and Sootopolis. When I first stepped into Slateport, I fell in love with the festive-sounding music. Even though there isn't too much to do there, other than shop, participate in a contest/battle in the Battle Tent, and stop Teams Magma/Aqua, it's still a memorable place. With Fortree, I love the concept of a city of tree houses, and the music's very relaxing. As for Sootopolis, I love the music and architecture very much, and I also like the unique way of first getting to it (i.e., via Dive).