。
That's what she dislikes most of all.
Literal
She [topic-は] it [subject-が] most disliked is.
もっとも (the kana form of 最も) is a slightly bookish superlative adverb, more written-feeling than the conversational 一番. The construction ~が嫌い uses が rather than を because Japanese treats emotional states like 嫌い・好き as adjectival predicates describing the experiencer's relationship to a stimulus — the disliked thing is grammatically a 'subject' of the dislike-state, not a direct object. This が-marking applies to a small, high-frequency set of predicates: 好き, 嫌い, 上手, 下手, 苦手, 得意, 怖い, 欲しい — worth memorizing as a group.