Which city is center of the whole world?

There is no city in the center of the world. The center of the Earth is an iron core. But on the surface, there is no such thing as a geographical center point. This is because since the Earth is a sphere, you can only split the Earth in half one way going north and south (the Equator). However, you can split the Earth in half infinite ways going east and west (the Prime Meridian). So truly, the Prime Meridian could be placed anywhere on the planet, which means longitude could be altered.