Please note I'm not a teacher.
After superlatives, in is often used with a word referring to a place or group.
the best in the world/ in the team / in the family/ in the hotel / in the class etc.
of is often used with periods of time, plurals and pronouns
the best of all / of us / of them / of the men etc.
You can also say the world's best / biggest / fastest etc.