Whomever
You use "whomever" because it is the direct object of the verb "trust."
"Whoever" is a subject pronoun. "Whomever" is an object pronoun. That means "Whoever" performs the action of verbs. "Whomever" receives the action of verbs. If the pronoun were performing the action of trusting, it would be "whoever," but it is not performing the action of trusting but is receiving the action of trusting that the subject "we" is performing.
By the way, take care not to assume the first word in a sentence is the subject. Subjects perform the action in a clause. It is clear that "we" is performing the action and that the sentence is using an inverted structure that places the direct object (i.e., "whomever") of the subject-verb (i.e., "we trust") before the subject-verb (i.e., "whomever we trust") instead of afterwards (i.e., "we trust whomever"). Whether the pronoun comes before or after "we trust," it is nonetheless the object of the verb, not the subject.
Moreover, even though the phrase "whomever we trust the most" is the subject (one of them, anyway) of the ensuing verb "is," that has no bearing on what pronoun, whoever or whomever, you are to use. You must look at the grammar of the phrase since it is the entire phrase, not the individual pronoun, that is serving as the subject of "is." Within that phrase, grammar calls for "whomever" since it is the direct object of "trust."