Here’s a concise guide to the specific episode and context for in the One Piece anime. Key Episode Episode 473 – "Wrapping Up the War! The Red Hair Pirates Appear!" (The Haki burst actually occurs in the latter half of Episode 473 and extends into Episode 474.)
However, the most famous cited for this moment is Episode 474 (title varies: "Execution of the Admiral" or "Conqueror's Haki" depending on the sub/dub). luffy conqueror's haki marineford episode