。
She's one of the world's finest ballerinas.
Literal
She [topic-は] world [in-で] most outstanding ballerinas [possessive-の] one is.
世界で最も優れた means 'the most outstanding in the world' — 最も (もっとも) is the formal superlative marker meaning 'most,' and 優れた (すぐれた) means 'outstanding, excellent.' ~のひとり means 'one of' — literally 'one person of.' バレリーナ is a loanword from Italian/English.