Shanghai's Urban Area is still the largest in China. If you take Metropolitan Areas, then Guangzhou, whose Metropolitan Area swallows up nearby Shenzhen (pop 12 million), would be considered the largest.
An interesting one with South Africa's 3 capitals, which are zero, 367 and 1263 km from Johannesburg. (The "zero" is if you accept citypopulation.de's view that Pretoria is a part of Johannesburg, but from city centre to city centre, you're looking at around 50 km). Will put that as a note into the instructions.