Which French cathedral has the highest nave vault of any completed Gothic church?

Amiens Cathedral has the highest nave vault of any completed French Gothic cathedral.

Its nave vaults rise about 42.3 metres above the floor, an extraordinary height for a medieval masonry structure. Construction began around 1220 after a fire destroyed the earlier church, and the building was developed in the High Gothic style by Robert de Luzarches, Thomas de Cormont and Regnault de Cormont.

Amiens followed the structural experiments of earlier cathedrals such as Chartres, but its three-level elevation and flying-buttress system allowed builders to create a taller, brighter interior. Its vaults exceed those of Chartres and Reims. The cathedral is also celebrated for its immense volume—about 200,000 cubic metres—and its extensive sculpture.

The main qualification concerns Beauvais Cathedral. Its choir vault reaches roughly 47.5 metres, higher than Amiens, but Beauvais was never completed as a full cathedral: its nave was barely begun. Thus Amiens is the standard answer when the question specifies the highest nave vault in a completed French Gothic church.