The critics declared the play a winner. what kind of complement is indicated by the bolded word?

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.