Answer :

Contrary to what many people would think, It's actually Tokyo, Japan that is the most populated city in the world. It was 37million+ but is more probably more closer to 38million but sadly, the city and country population is going down. (Ya, I'm mostly of Japanese blood) Other heavily populated cities are often just in the late 20million(Beijing, Shanghai, Seoul, Delhi)
The most populated city in the world is Tokyo, Japan, at almost 38 million. However Japanese women have a new trend where they ditch their parents and stay single as long as possible. This has caused the population to drop rapidly. Soon New York City will overtake Tokyo