。
She is a rare beauty.
Literal
[topic-は] rarely-seen beautiful woman [copula-だ].
まれにみる ('rarely seen, exceptional') is a fixed expression — まれに ('rarely') + 見る ('see'), pre-modifying a noun: 'exceptional, of a quality you rarely see.' まれに見る美しさ ('exceptional beauty'), まれに見る才能 ('exceptional talent'). Fundamentally a compliment marker that frames the quality as too good to be common. 美しい is the more elevated/literary word for 'beautiful,' compared to the everyday きれい. 美しい女性 has the slightly formal register of an admiring observation rather than casual chat.