best friend vs friend

best friend and friend both are nouns.

Word	Noun	Adjective	Verb	Adverb
best friend	Yes	No	No	No
friend	Yes	No	No	No

As nouns, friend is a hypernym of best friend; that is, friend is a word with a broader meaning than best friend:

best friend (noun)	friend (noun)
the one friend who is closest to you	a person you know well and regard with affection and trust
	a person with whom you are acquainted
	an associate who provides cooperation or assistance
	a person who backs a politician or a team etc.