The most ... in or of

Following the superlative (the most), we say in + singular words for places or groups of people. (in the city, in the world, in my class, in the company, etc.)

Example

• She is the best student in the class. ✅
• She is the best student of the class. ❌
• He’s the tallest in the family.
• Messi is the best player in the world.

And we use of for periods of time or a number of people. (of the year, of my life, of my brothers, of the students, etc.)

Example

• It was the best day of my life.

References

• Comparative and superlative adjectives and adverbs - Test-English