It's just been brought to my attention via randomly lookong through Bulbapedia that Ash's longest winning streak is 70 episodes, a large chunk of XY, including all of Kalos Quest.

This winning streak apparently started in Battles in the Sky! (XY036) (the episode where Fletchling evolves) and ends in A Meeting of Two Journeys! (XY106) (the episode where they all meet Alain)