。
She is the most important person to him.
Literal
She [topic-は] him [for-にとって] most important [adj-な] person is.
~にとって ('to (someone), from (someone's) point of view') is a perspective-marking compound particle — 'as far as X is concerned, in X's view.' Common in evaluative statements about importance, value, difficulty: 私にとって ('to me'), 学生にとって ('for students'). 最も is the formal/written intensifier 'most'; in casual speech, 一番 is more common. たいせつ (大切) is written here in kana, slightly softer feeling.