Huaqiangbei is the largest of its kind, without a doubt. There's areas designated to each kind of component, device, or service. I didn't know that it was possible to upgrade the capacity of an iPhone (by de-soldering flash memory chips and reprogramming firmware), but there are some very talented people in Shenzhen who do just that.
Wandering around helped me to get an idea of what was available. Sitting on the metro gave me time to reflect, and type up notes on my phone about different connectors. I compared voltages, imagined adaptors, and then went and bought them or built them. Although it didn't start (or end) there, my pockets grew significantly thanks to Huaqingbei.
https://news.ycombinator.com/item?id=21533819
By comparison, when I went to Akihabara in 2012, it was a lot cleaner and tidier, but more expensive and somewhat smaller. Taipei has Guanghua, which isn't bad either, but most of the shops are just in one mall. Kaohsiung has 2 streets (Changmingjie and Jianguolu) for components and finished products, respectively.
What I found to be most useful was checking Taobao beforehand, and sending messages to sellers using Aliwangwang or WeChat. Then I could get the address, go to their store, test it, and pick it up in person.
I used offline maps and GPS coordinates to be certain about addresses, and YouLing (my own app) for instant translation. Now WeChat's translator is probably good enough.
What I'd like to know about modern Shenzhen is how the cashless society has affected sales. Does everyone use Alipay?