While the legendaries this Gen were not up to par with Gen 4, we have some good ones. Meloetta is one of them. It's ability to take down any defensive foe (excluding Shuckle, Bronzong, and Dusknoir) makes it a valuable addition to OU, where teams can be using Blissey one minute, and Foretress the next. I have a few movesets for it.
Ability: Serene Grace
EVs: Attack, Sp Attack, Speed Nature: Hasty
Ancient Song: ALlows this Pokemon to be a mixed attacker.
Close Combat: A powerful Fighting attack for use with Step forme. Death to Blissey!
Shadow Ball: Deals with Ghost types, and has a good Sp Def lowering chance.
Psychic/Stone Edge: Which one, STAB or type coverage?