What type of adjective is the word best?

It can be either noun, verb, adjective, or adverb. However, in some uses, the noun can be considered an adjective form. i.e. best one.

Examples:

(noun) "Of all the candidates, he was the best." (i.e. best candidate)

(noun) " He always tries to do his best."

(verb) "We will try again this week to best our rival school.

(adjective) "It was the best result that we could hope for."

(adverb) "Father knows best."

(adverb)"The system performs best when run separately."