。
She's my best friend.
Literal
She [topic-は] my best-friend is.
親友 ('best friend, close friend') is a Sino-Japanese compound built from 親 ('intimate, parent, close') + 友 ('friend'). The 親 here carries its 'close, intimate' sense rather than the family meaning. Distinct from 友達 (general 'friend'), 友人 (formal 'friend'), and the more demonstrative 大親友 ('very best friend'). The polite ~です raises the register slightly.