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She's the most beautiful model I've ever seen.
Literal
She [topic-は] until-now seen [experience-こと]-also not-existing pretty model is.
見たこともない ('haven't even ever seen') stacks the experience pattern ~たことがある ('have had the experience of') with the additive も, packing in 'have not even once seen.' The whole noun-modifying clause 今まで見たこともないきれいな precedes モデル, painting a relative clause that translates into English as a 'most ... ever' superlative. Japanese routinely builds this kind of layered prenominal modifier, where the noun comes last and the entire description leads up to it. モデル is a katakana loanword in the fashion sense.