Peter is (happier,the happiest)boy that i know

Peter is the happiest boy that I know.  Питер самый счастливый мальчишка, которого я знаю.

Adjectives have three degrees of comparative. When adjectives have one or two syllables we form their degrees with the help of the suffixes - er and - est. (Tall taller the tallest). Y is turned into i after consonants. Happy happier the happiest.  We use the definite article with the Superlative. 

When adjectives have more syllables we use more and the most. (Interesting more interesting the most interesting).