greatest vs superlative

greatest is not a noun while superlative is a noun.

Word	Noun	Adjective	Verb	Adverb
greatest	No	Yes	No	No
superlative	Yes	Yes	No	No

As adjectives, greatest and superlative are synonyms defined as:

Other synonyms of greatest include sterling.

greatest (noun)	superlative (noun)
	an exaggerated expression (usually of praise)
	the superlative form of an adjective or adverb
	the highest level or degree attainable; the highest stage of development

greatest (adjective)	superlative (adjective)
highest in quality	highest in quality