Any of the Treaty Ports could have been held for longer, I suppose. Hong Kong itself was comparatively backwater until Shanghai helpfully exited the race via Communist takeover.
Shanghai is probably the least likely city to be taken over by Western powers, since it was probably the richest city actually in the Chinese Empire/Republic, and therefore would become a natural focus for liberation. Same with Canton and Tianjin. Although TBH, I don't think Communist China would really use military force to retake colonies, considering their precarious international position. Nationalist China would probably have been more aggressive in doing so.
Amoy (Xiamen), Fuzhou, Swatow (Shantou) and Swabue (Shanwei) are possible contenders, especially if Hong Kong doesn't turn out that well ITL. Dinghai on Zhoushan Island, as mentioned, is also possible. But their viability does require the Western powers to actually annex the territory, as opposed to nebulous international settlements and the like, which will be lost as soon as the efficacy of 'gunboat diplomacy' fades.
Haikou, Beihai and Guangzhouwan could have been kept simply because they were too irrelevant by the end of WWII to risk war over, kind of like Macau.
The whole of Taiwan could have conceivably become a massive Hong Kong, though unlike HK it would probably be granted independence rather than being returned to China after the colonial period.