The copula should always agree with the subject, not with the complement.
The same order of words is required by the copula with whatever kind of complement.
Each is a sequence of noun-copula-adjective-infinitive verb.
The copula is present if the superlative is used predicatively.
British Dictionary definitions for copula
noun (pl) -las, -lae (-ˌliː)
a verb, such as be, seem, or taste, that is used merely to identify or link the subject with the complement of a sentence. Copulas may serve to link nouns (or pronouns), as in he became king, nouns (or pronouns) and adjectival complements, as in sugar tastes sweet, or nouns (or pronouns) and adverbial complements, as in John is in jail
anything that serves as a link
(logic) the often unexpressed link between the subject and predicate terms of a categorial proposition, as are in all men are mortal
C17: from Latin: bond, connection, from co- together + apere to fasten