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
"Craziest" is an adjective in its superlative form.
Greatest is the superlative adjective of great.
superlative adjective
adjective (superlative form)
An adjective in its superlative form.
Darker is a comparative adjective
Scariest is the superlative form of the adjective scary.
Largest is an adjective. It's the superlative form of large.
Deepest is an adjective. It's the superlative form of deep.
Biggest is an adjective. It's the superlative form of big.