A century ago the world’s biggest cities were New York City, London, and Paris. They were the centers of the global population, however as the population of the developing world has skyrocketed in the last century so has the population of megacities in the developing world. In 2015 most of the world’s biggest cities can be found in India, China, and the African continent.

The European colonial era begins with the voyage of Columbus in 1492. At this time the capital city of the Aztec Empire, called Tenochtitlan, was by most estimates the most populous city in the world. Loose estimates, which are all that are possible, place the population of Tenochtitlan at around 200,000 people.

If the current trend continues most of the world’s most populous cities will be found in Asia, specifically South and East Asia, in the year 2025. Cities like Beijing, Shanghai, Tianjin, Delhi, Mumbai, Kolkata, Dhaka, and Guangzhou are growing at dramatic rates when compared to much of the rest of the world and will soon eclipse their western counterparts.

A “megacity” is a city that has a very large, and growing, population. Megacities are typically found in the developing world. They are characterized by high natural population growth, as well as high rates of immigration. They generally suffer from extreme pollution and poverty and will usually feature widespread slum settlements to house the working masses.

The term “megacity” is used to describe a city that has an extremely large, and growing, population. In the previous half-century megacities have been emerging all over the developing world as the population of urban environments like Mumbai, Shanghai, Lagos, and Dhaka explodes.

According to most estimates Tokyo is not just the most populous city in Asia, it is the most populous city in the world. Almost thirty million people live in Tokyo.

Istanbul, the capital city of Turkey, lies on the Bosphorus Strait, at the border between Europe and Asia. Due to its strategic location Istanbul has been of great significance for several centuries and has often been fought over between regional powers.

A "megacity" is an urban or metropolitan area which has a population over 10 million people. 

Here are the populations of the cities from this question: 

Tokyo: 37,900,000

Seoul: 26,100,000

Shanghai: 25,400,000

Jerusalem: 809,112

London: 8,540,000

Tokyo is currently the most populated city today. Jerusalem does not even reach 1 million so it can not be classified as a megacity. 

Jerusalem, Israel is not a Megacity. It only has a population of about 800,000. Though it is a very important city in the world, as it is often fought over and boasts a significant historic site of three of the major world religions (Judaism, Christianity, and Islam). It's population is not currently growing, probably due to violence and war as well as the religious focus of the city rather than economic growth.

Tokyo, Japan is a Megacity and is the most populated city in the world, boasting a population of 38 million.

New York, USA is a Megacity and was one of the first two Megacities in the world. The current population of New York is 23 million.

Jakarta, Indonesia is number 3 in the world in population rank, coming in behind Tokyo and Shanghai. Jakarta has a population of 31 million.

Paris, France is one of the smaller Megacities in the world, but nevertheless it is a Megacity. Paris has a population of 12 million.