I think any Canadian cities like Toronto or Vancouver or Dublin, Ireland are best contenders for the title and here is my reasoning for it. You need to wait for about a year to get a H1 visa to work in SV or work in the same firm for over a year elsewhere to get an L1 visa. So essentially SV today is a representation of diverse global talent who could get the visas, which is a very limited demographic. Historically SV did have an information advantage but if information were truly open, which I think is what the world is moving towards, I would assume any country with access to true diverse workforce without any limitations on workforce mobility will have an upper hand. And you already see accrual of talent to some extent in these cities. Nowadays whenever top 5 companies hire new talent from countries other than US, they usually bring them directly to one of these cities outside US. Competitors open up shop in the same cities because of easy access to talent.

I can see a world where ideas and startups increasingly coming from these above cities. And for the same reason, I don't think any of the Chinese cities pose a threat to SV. If anything Chinese cities can produce a Chinese version of the product but it will be hard to produce a global firm without access to diverse global talent.