What is the subject of the following sentence?: To everyone's dismay, the movie did not have a surprise ending.
Movie is the simple subject
The subject is what/who is being talked about in the sentence. The sentence is talking about the movie not having a surprise ending. The subject must be a noun or a pronoun.
It might be tempting to say that "everyone's" or "dismay" is the subject, but both words are parts of a prepositional phrase (to everyone's dismay). Subjects are never parts of prepositional phrases.