We use the superlative form when we talk about the biggest, or the most important thing or person among a group of things or people, like this:
The Empire State Building is the tallest building in New York City.
Jack is the best baseball player in our school.
Tokyo is the biggest city in Japan.
When you want to mention that the thing or person is a member of a group containing the biggest, or the most important things or people, you can use one of. We use one of + adjective + plural noun, and we use this combination with a singular verb, like this:
The Empire State Building is one of the tallest buildings in the world.
Jack is one of the best baseball players in our city.
One of the biggest cities in Asia is Tokyo.