I doubt it's even Guangzhou now as they have seriously cracked down on the illegally residing Africans and have started gentrifying the areas where they used to live and trade. If it's not Guangzhou or Shanghai; or the other tier 1's Beijing and Shenzhen, it could be Nanjing as a lot of foreigners are moving there now for some reason.

I also wouldn't be surprised if the likes of Chengdu. Chongqing and Kunming actually have the most; but many would be unregistered according to such counts, as they are often transient backpacker types possibly working illegally on tourist visas. Yiwu also for similar reasons, although that's more about the Arab guys regulary popping over to buy wholesale products.

Also, one wonders about the Northern cities where there may be a large Korean population. Beijing was once said to have at least 100,000 Koreans living in or around the slightly far out suburb of Wangjing alone, so the likes of Qingdao, Shenyang and Dalian could actually have more foreigners present at any given time despite our Western eyes not easily noticing it; despite recent problems faced by Korean businesses in China due to political issues.