A leading player or star (a troupe's main performer / box-office star) — a four-character idiom used for someone's top attraction.
あの劇団の一枚看板はベテランの女優だ。
That troupe's leading player is a veteran actress.
新作の主演が一枚看板として宣伝されている。
The new production's lead actor is being promoted as the show's marquee star.
映画館は地元出身の俳優を一枚看板にして観客を呼んだ。
The cinema used a local actor as its marquee star to draw audiences.
One's best item or chief attraction — the single thing someone or a shop relies on to show or promote themselves.
この店の一枚看板は手作りのカレーだ。
This shop's signature item is its homemade curry.
会社の一枚看板製品が不調で業績に影響が出た。
The company's flagship product performed poorly and affected its results.
彼の一枚看板はユーモアのある司会ぶりだ。
His standout quality is his humorous MC style.