Assuming that the bolded word is winner, the correct answer is object complement.
The word winner refers to the word play - and given that the word play is the direct object of this sentence, then the word winner will be its complement - it will complete it, and answer the question What did the critics declare the play to be?
It cannot be a predicate nominative because it is not preceded by a verb. It cannot be a direct object because that is the play. It cannot be a predicate adjective because it is a noun, not an adjective.