I would really like to battle the highest level/most challenging trainer in Pokémon X
So.. who's the strongest trainer you have found? I know Dianthas Gardevoir is level 68 and Zygarde is level 70. That was the question ^^
For as far as I've seen the game the champion is the stongest trainer in Kalos. Furthermore Zygrade is the strongest in-game Pokémon (like you already said).
Actually, the highest level wild Pokemon are indeed Zygarde and Mewtwo (70).
EDIT: The Kanto bird trio is lvl 70 too (Thank you SparkyBlaziken :) )
But the actual highest level Pokemon trainer is Diantha, when you rematch her in the battle chateau, if you wrote a 'Black writ of challenge'. Her Pokemon will be lvl 80. (If you didn't they will just be 60)