For its size and importance in the world, I have to confess that I have spent less time in China than you would expect. That said, I have made several trips to the Middle Kingdom and find it a fascinating place.
For a variety of reasons, Shanghai is the mainland Chinese city with which I am most familiar. Read more about my experiences in what is arguably the most international of Chinese cities here.
My experience with Beijing is limited to only a single trip. At some point I can share more about the city and will hopefully return soon so I have a more-informed opinion
In the late 1990s I lived briefly in Hong Kong and it remains one of my favorite cities in the world. Read about some of my experiences here.