I have to say I didn't really have an interest in visiting China until I saw pictures of Chongqing. I spent a whole night one day watching walking videos in that city and vertical nature of it is just incredible.
My personal favorite was Beijing. Honestly, it felt like stepping into another world, with a vibe unlike anywhere else I have been, even compared to Shanghai or other East Asian cities like Tokyo. The people were incredibly warm and friendly, the street food was outstanding, the environment felt almost otherworldly, and the historical sites were phenomenal. Getting lost in the hutongs felt like its own adventure, the punk culture was alive and thriving, and the art district was the best I have experienced (even coming from New York City). If you skate, it's a paradise with spots everywhere you turn. Public transportation and taxis were also super affordable, clean, and efficient.
Shanghai was cool too, but it felt much more familiar. At its core, it reminded me of Manhattan, just more intense. The people there also gave off a similar vibe to New Yorkers. If you are into electronics, it's a great place to explore, although I hear Shenzhen is really the true hub for that.
Baotou felt a lot more desolate but also very peaceful. I spent most of my time there on a farm and hiking around the Gobi Desert though.
One thing to be aware of is the air quality in Beijing and Shanghai. On bad days, you would come back with black snot. I was a smoker at the time, which did not help, but even non-smokers experienced it. There always seemed to be random particles or even feathers floating in the air lol.
As someone who skates, I found the police and security surprisingly relaxed. They would usually let you skate for a while before politely asking you to move along. The only unsettling thing I witnessed was a group of officers with SMGs escorting someone out of Tsinghua University.
The saddest thing I saw was the desperate poverty on the outskirts of Beijing. There were large piles of trash, open sewage, and pollution. At one point, a concerned local pulled over and told us to turn around because of violent protests between farmers and the government up ahead. The outskirts of Shanghai showed similar poverty, though without the protests.
That said, aside from the outskirts and days of bad air quality, all three cities were incredibly clean, especially compared to New York.
This was all about 15 years ago, so I imagine a lot has changed since then.
Still, I would absolutely recommend going. If you can, spend time in places that feel unfamiliar, especially Beijing. It's an experience you will never forget.