Грамматика русского языка. Часть 7. Имя прилагательное. Глава 14. Превосходная степень. Russian Grammar. Part 7. The Adjective. Unit 14. The Superlative.

lf we say something is the most interesting, smallest, best we are using the superlative form of the adiective.
The superlative is very easy to form. Simply put the adjective самый in front of the the adjective and noun you are describing. There is no short form of the superlative (so it can be used predicatively and attributively - before or after the noun):

Самый серьёзный фильм   (the most serious film)
Этот фильм самый серьёзный   (this film is the most serious)

Make sure that the adjective самый agrees in number, gender and case with its adjective and noun:

Я изучаю самый красивый язык   (I am studying the most beautiful language)
Я читаю самую серьёзную книгу   (I am reading the most serious book)
Мы живём в самом приятном районе   (We leave in the pleasantest region)

Самый can be used with the comparatives лучший and худший to mean best and worst, or they can just be used as superlatives in their own right, so:

Это лучшая идея and это самая лучшая идея both mean it’s the best idea.

The same applies to младший (younger/youngest) and старший (older/oldest).

To say “the most... of (e.g. one of the most interesting books), use the preposition из:

Одна из самых интересных книг   (one of the most interesting books)

A very small number of adjectives fonn their superlative with the ending айший or ейший. The most useful are in phrases such as:

У меня нет ни малейшей идеи.   I haven't the slightest idea.
Чистейший вздор!    Utter rubbish!
Ближайшая станция метро.   The nearest metro station.