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)