Sunday clothes vs Sunday best

Sunday clothes and Sunday best both are nouns.

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

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

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

Sunday clothes (noun)	Sunday best (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