Any opening we create out of a game
longest opening
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If you define the longest opening as a position with the highest move count that occurred in more than one game, then it is probably the Marshall Variation C89 of the Ruy Lopez. The Najdorf Sicilian B99, the Sveshnikov Sicilian B33, and the Grünfeld Indian Defence D89 also are contenders. To be sure you would have to query a data base.
Maybe Sicilian Defence
whats the longest opening