Sunday best vs Sunday clothes

Sunday best and Sunday clothes both are nouns.

Word	Noun	Adjective	Verb	Adverb
Sunday best	Yes	No	No	No
Sunday clothes	Yes	No	No	No

As nouns, Sunday best and Sunday clothes are synonyms defined as:

Sunday best and Sunday clothes: the best attire you have which is worn to church on Sunday

Sunday best (noun)	Sunday clothes (noun)
the best attire you have which is worn to church on Sunday	the best attire you have which is worn to church on Sunday