what is the most populated city In the world

Depends on what you mean by city.

If you mean what is the largest city by the amount of people in its city limits, the answer is Mumbai, with a population of 13 073 926.

If you mean city as in the amount of people in the metropolitan area, it is Tokyo with a population of 32 450 000.

If you mean city as in by the urban area, it is Tokyo/Yokohoma with a population of 34 450 000.

(Note that if you go by the first category, Tokyo only comes 12th)