Pokito, you and Sage have the right instinct, and in fact, the other way would be wrong.The drums are her favorite instrument.
Her favorite instrument is the drums.
but notX Her favourite instrument *are the drums.
(Though perhaps a mistake that some native speakers would occasionally make, especially in longer sentences with more abstract words.)
This is a sentence in the classic LEO categoryThe problem is the neighbors.
The neighbors are the problem.
vs.Die Nachbarn sind das Problem.
Das Problem sind die Nachbarn.
which originally came up in a thread on a different singular/plural topic:related discussion: variety with plural or singular verb?
There have been several other discussions on singular/plural agreement when two nouns or pronouns are joined by linking verbs. English treats the noun or pronoun before
the verb as the subject, so the verb has to agree with it. In German, the noun after
the verb can be considered the subject, where in English it would only be the complement.