one and only vs nonpareil

one and only is not a noun while nonpareil is a noun.

Word	Noun	Adjective	Verb	Adverb
one and only	No	Yes	No	No
nonpareil	Yes	Yes	No	No

As adjectives, one and only and nonpareil are synonyms defined as:

Other synonyms of one and only include matchless, one, peerless, unmatchable, unmatched, unrivaled, unrivalled.

one and only (noun)	nonpareil (noun)
	colored beads of sugar used as a topping on e.g. candies and cookies
	a flat disk of chocolate covered with beads of colored sugar
	model of excellence or perfection of a kind; one having no equal

one and only (adjective)	nonpareil (adjective)
eminent beyond or above comparison	eminent beyond or above comparison