After superlative adjectives, we do not usually use of with a singular word referring to a place or group.

She is the most beautiful woman in the world. (NOT She is the most beautiful woman of the world.)

I am the happiest woman in the world. (NOT I am the happiest woman of the world.)

He is the best player in the team. (NOT He is the best player of the team.)

But of can be used before plural nouns and pronouns.

She is the fastest player of them all. (NOT She is the fastest player in them all.)

Of can also be used before singular quantifiers like lot and bunch.

He is the best of the lot.

Infinitives after superlatives

An infinitive structure can be used after a superlative. This has the same meaning as a relative clause.

Who is the youngest person to become the President of the US? (= Who is the youngest person who has become the President of the US?)

This structure can also be used after words like first, last and next.

She is the first woman to win the Nobel Prize for Literature. (= She is the first woman who has won the Nobel Prize for Literature.)