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顔面国宝 ganmen kokuhou
That face should be state-protected.
What does ganmen kokuhou mean?
A national treasure, but it's a face. Reserved for beauty so unreasonable that the government should probably get involved.
Example
友達の証明写真、顔面国宝だった。
My friend's ID photo was a certified national treasure.
Vibe
Who you can say this to, and how it lands.
Friends only Casual. Natural with friends, out of place at work or with strangers.
The story behind ganmen kokuhou
国宝, national treasure, is a real legal designation in Japan, applied to castles, statues and scrolls that the state has decided to protect. Sticking 顔面, face, in front of it is a joke about scale: this face has passed beyond compliments and into cultural property, and somebody ought to be filing the paperwork.
The exaggeration belongs to a family. Japanese fandom loves a bureaucratic superlative, and magazines have run 国宝級 rankings for handsome faces for years. Note how the praise works though. It says nothing about the person, only about the face, and it is usually shouted at a photograph rather than said to anyone in the room.