Nouns modified by adjectives in the superlative degree normally have the article the.

She is the most beautiful woman I have seen. (NOT She is most beautiful woman I have seen.)

It is the best book I have read. (NOT It is best book I have read.)

Who is the fastest runner in the world? (NOT Who is fastest runner in the world.)

Who is the youngest woman to scale Mount Everest? (NOT Who is youngest woman to scale Mount Everest?)

Superlative adjectives in the predicative position can be used with (formal style) or without (informal style) the.

This site is the best. (Formal)

This site is best. (Informal)

Superlative adverbs is sometimes used without the in an informal style.

Who runs the fastest? (Formal)

Who runs fastest? (Informal)

The is not used with superlative adjectives or adverbs in predicative position when we compare the same quality of a person in different situations.

I work best in the evening. (NOT I work the best in the evening.) (Here my work is being compared in different situations.)

I have got a lot of friends, but she is the best. OR I have got a lot of friends but she is best. (Here she is being compared with other friends I have.)