Of this list I'd have to say Greece or Rome, maybe a tie between the two. I would say Celts as well, but most of the culture we know about them these days comes from more recent times than "ancient".
English
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[I mean yeah, we’re finally learning about the celts after however long ago they were around. Thanks British empire, your racist ass has know limited our knowledge of ancient history.]
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It's so true. Which is ironic because of how big of a melting pot Britain became culturally. They squelched a lot of cultures, but in the end those cultures changed the Empire as well.