。
She's the most beautiful of all the girls.
Literal
She [topic-は] other any girl [than-より-also-も] beautiful.
This is the standard 'X-est' superlative built from comparatives: 'more beautiful than any other girl' = 'the most beautiful of all.' ほかのどの ~ よりも is the canonical pattern: ほかの ('other'), どの ('any'), よりも ('than even, more than'). The trick is that Japanese comparatives don't have dedicated superlative morphology like English '-est' — instead the superlative reading emerges from comparing against 'all others.' The variant 一番 + adjective is a more literal way of saying 'the most.'