The word "superlative" can be a noun, adjective, or adverb depending on how it is used in a sentence.
superlative adjective
adjective (superlative form)
Costliest is an adjective. It is the superlative of costly.
"toughest" is an adjective ( the superlative form of "tough" )
Strongest is an adjective. It's the superlative form of strong.
an adjective (the Superlative).
it is superlative
superlative adjective
Greatest is the superlative adjective of great.
"Craziest" is an adjective in its superlative form.
adjective (superlative form)
An adjective in its superlative form.
Scariest is the superlative form of the adjective scary.
Darker is a comparative adjective
Largest is an adjective. It's the superlative form of large.
Deepest is an adjective. It's the superlative form of deep.
adjective a superlative adjective.