“Justin is the best” is not a full sentence, and as such does not have a part of speech. If it were part of a sentence such as “My friend Justin is the best,” “Justin” would be a noun (proper) and “best” would be an adjective.
But we can assume that the keyword is used to express a positive opinion about someone named Justin. It could be used to describe his personality, skills, or achievements. In this sense, the phrase “Justin is the best” is a way of praising him.