I'd say either Undella Town, Village Bridge, or Mistralton City. I love Undella town because of the beaches and because if you go south you get to a few waterfalls which is so beautiful. I also like the Village Bridge because it reminds me of Italy(though i've never been there, LOL) with it's beautiful old bridge, the river beneath it, and the many musicians playing. Plus it has that sandwich shop, and I LOOOVEEEE sandwiches! I also like Mistralton because of the awesome planes and jets there(the runway just rocks socks too). It also rains there, and the gym leader specializes in Flying types which are my favorite type!
I guess if I had to choose one, it would be the Village Bridge mainly because of the sights and because of the musicians. But I would still like to visit the other places too..