。
She is the most important person to me.
Literal
She [topic-は] me [for-にとって] most precious person is.
~にとって ('for / from the perspective of') marks the frame within which a judgment holds — what is true 'for me' or 'from the student's perspective.' Distinct from the dative に: 私に大切な人 would be odd, but 私にとって大切な人 is natural. もっとも (副詞 'most') is the formal/written equivalent of いちばん, frequent in writing and elevated speech. 大切 ('precious, important, valued') is a particularly warm na-adjective — softer and more emotional than 重要 ('important' in a more cool, evaluative sense).