A predicate nominative because "one" follows the verb "to be- was (past tense). A predicative nominative alwasys follows the verb to be in al of its forms and one should view it as "equals", as in, Langston Hughes equals on of the greatest poets of the Harlem Renaissance. A predicative adjective must modify the subject and is linked with a verb, but in this sentence "one" does not modify Hughes. Another trick for remembering is that if you were to switch the order of the sentence, it would still make sense; "One of the greatest poets of the Harlem Renaissance is Langston Hughes."
