star performer; leading player; box-office draw
Originally from theater, referring to the single main attraction or star whose name is on the signboard. Now used for any field's standout performer.
He is the star performer of this theater company.
あの歌手は事務所の一枚看板として活躍している。
That singer is active as the agency's leading star.
one's best asset; sole strong point; trump card
Extended figurative use: the one thing or person you can proudly show off, often implying it's the only notable asset.
この料理がうちの店の一枚看板です。
This dish is our restaurant's signature item.
彼の一枚看板は英語力だ。
His one strong point is his English ability.