Mine would be Nimbasa City, I find myself always hanging around there most of the time, and usually when I start up my save I'm there. I not only absolutely love the music that goes with it, I also tend to love going into the Stadiums and the Musical there too, and because it's closest to the Join Avenue, I go there alot, and my Avenue's on a pretty high level because of it.
But besides that, my second favorite would be Humilau City, mostly because of how upbeat it looks, and how pretty it is with the sun on it, and I personally like the water there too, and how there are houses on boards above it and yeah.