Which mega evolving pokemon is most helped by its non-mega ability, assuming that it is meant to hold and use its mega stone. For example Medicham's pre-mega ability is pure power, which is a really useful ability if it doesn't mega evolve. But as a pre-mega ability it sucks because Medicham will always mega evolve before it attacks. In face you're better off running telepathy even in singles just in case it switches into a foul play. On the other hand Clear Body is a pretty decent ability most of the time, although its nothing special. But it lets Metagross and Diancie switch into intimidate and sticky webs without lowering its stats which makes really nice as a pre-mega ability.