Happy is the man who finds wisdom.
Sometimes there is only one possible subject, which makes that subject easier to find. As should be obvious from this example, the subject is not always in front of the verb.
The subject of the copula is the thing that possesses some state. The complement of the subject is the thing that represents that state. In the biblical example above, the adjective "happy" represents an emotional state. It must be the complement, leaving "the man who finds wisdom" as the subject who possesses some happiness.
The best part of the movie are the quirks.
In this case, both the subject and the complement are noun phrases. Grammatically, either phrase could play either role. We need to look at the semantics. Which one of these referents can more sensibly possess the other as a state?
To my eye it seems more likely that the "quirks" possess a state and that "the best part" represents or characterizes that state.
The other arrangement is possible. We could discuss several parts of the movie. We might decide that the best part is the quirks, the worst part is the frequent bits of wooden dialogue, and one mediocre part is a long stretch of silent cinematography. In a context like this, "the best part of the movie" is a more sensible subject.
This sentence offers another way to be sure that subject and verb agree: let the subject and its complement both share grammatical number.
The best parts of the movie are the quirks.
Here, the verb agrees with both noun phrases.