Mega Diancie

Mega Diancie is a monster among Mega Evolutions. This thing is one of the absolute strongest Rock Type Mega Evolutions, generally only really rivaled by Mega Tyranitar. However, between these two we have a tale as old as time: Mega Diance will generally have stronger DPS, while Mega Tyranitar will generally have better TDO. Regardless of individual matchups and competition, Mega Diancie is a very real competitor for the gold medal of the type. And if it ever gets a Fairy Type Fast Move, then it could genuinely be great there as well.

And to add even more fuel to the fire, Mega Diancie has a Rock Type Signature Move in the main series named Diamond Storm, meaning it may very well become even stronger someday.