Mong Kok is a district on the Kowloon Peninsula in Hong Kong. The area is defined by old and new buildings in highly compacted architecture. From what I’ve heard, Mong Kok is the most densely populated region in the world. Although Hong Kong is a very vertical city, there is plenty to see along the international streets, and street photographers that haven’t been there before will have a field day.
If you plan on visiting, I would go at night so you can see all of the brightly lit neon signs, particularly along Nathan Road. It’s right in the middle of Kowloon and very easy to reach and find via public transportation.