The most populated cities in the world are Tokyo, Mexico City and New York City.
My definitions for 'cities' here are actually the full built-up area around the city and I have included the suburban areas as well. I think this will make a fairer comparison of the cities as their own fullest entities.
Tokyo's Major Metropolitan Area, as reported in the Japanese Statistics Bureau, has 35.197 million people. Mexico's Metropolitan Area is home to 19.411 million people. Both cities are capital cities to more than 100 million people and both cities are in Organization for Economic Co-operation and Development (OECD), signaling that both nations are highly developed nations with a high degree of industrial and urban development.
However, because of Japan's better efforts industrialization and economic development as compared to Mexico, Tokyo is, of course, much more developed and advanced than Mexico City. Tokyo has the financial, political, economic, social, health and educational resources to meet the needs of the Japanese and even the world. Mexico City, as of date, do not have similar resources to do so.
New York City, the one-time greatest city of the world, is one of the three largest cities of the world. But it is not a political capital of United States and even its state. It seems to prefer to be an economic powerhouse, a beacon that shines throughout United States. As a result, it is now home of more than 18 million people. Though Sao Paulo and Mumbai are rapidly growing as well with only a little less people than New York, for now, New York City is indisputably one of the 3 most populated in the world.
I discover that the most populated cities in the world are not only strong economically, providing more jobs for their citizens than any other place in the world; they also provide the best opportunities for transport, culture and even sports. All roads and railway in eastern United States seem to lead to New York. All national roads and railway systems in Japan and Mexico converge in Tokyo and Mexico City respectively. In addition, rap and hip-hop music, J-pop, classical music, theaters, sumo, baseball (New York Yankees and Yomiuri Giants especially) are originated, or at their best, in the most populated cities in the world.
The most powerful cities in the world are powerful enough to move and shift the whole world, so they are worthy of note.