Queen's Gambit Declined: Baltic Defense

The Queen's Gambit appears in the earliest opening manuscripts (c. 1490, Göttingen). It anchored world-championship matches for a century — Capablanca–Alekhine 1927 was practically a Queen's Gambit theme event — and in 2020 a TV show made a 500-year-old pawn offer the most famous opening on the planet. It isn't even a real gambit: Black can't safely keep the pawn.