Mega Ampharos and there is absolutely zero room for debate. That slick yellow body, that bulbous red orb and that beautiful luscious hair all point to Mega Ampharos as the best Pokemon.
I don't get it. Mega Diancie is literally just a set of gray arms and a gray humanoid head stuck onto a pink rock dressed in a pink wedding gown or something. Not a single part of her convinces me to even consider her being the best Pokemon.