Meteor Mash is a better STAB and Zen Headbutt doesn't hit SE on any of the types that resist Meteor Mash. Psychic only hits Poison and Fighting SE and all of the OU Poison and Fighting types get hit nuetrally by Meteor Mash except for Lucario, Infernape, and Tentacruel and non of them have good Defense and they all get hit SE by Earthquake anyway. Earthquake does more to most Fire, Steel, and Electric types and ThunderPunch does more to Water types and Skarmory. Like I said before, Zen Headbutt is really only good for hitting Rotom-A.

Why not ue Zen Headbutt? It gets STAB...