May be the london is the most expensive city but it depends on the country and the lifestyle of the people living there. John
It must be the cities in the main land of China, do you know why?? Because the income of most people in China is very low ,but every thing in China is expensive,An 100 m² apartment cost about 100 million USD in Beijing ,but most people can earn only 600 USD per month, even spend their whole life, they can not offer for an apartment. A BMW760 cost about 100 thousands USD in the America,but in Beijing you have to pay 400 thousands USD for it, and the gasoline price is 1.3USD, own a car in the USA is very common, but own a car in China means you are really wealthy,and what more ,they don't have welfare, so medicine ,education and every thing on their own cost,everything they need money! money!! In a word, consider about the income ,maybe Beijing is the most expensive city inthe world.
I don't know exactly which exactly but I know here in NYC the prices are outrageous. We're in at least the top 3.
I think Paris is the most expensive city in all over the world. May be i m wrong but its my personal experience