I'm tied between Castelia City and Nimbasa City. Castelia is by far of the most beautiful cities I've seen in a Pokemon game. The first time I stepped into it, I was literally blown away. I love the skyscrapers, multiple alleyways, and rotating and swooping 3D camera views , they all left me in awe the first time visiting. And Nimbasa is just a fun city with lots to do. It has a ferries wheel the people you ride with have awkward conversations with you , an awesome looking amusement park/runway gym, the Musicals, and many places to battle. The Big Stadium and Small Court are probably my favorite place there cause I can battle trainers everyday and it's perfect for grinding and training.
Tied for second are probably Virbank City, Humilau City and Undella Town.