I would say Mewtwo would have a nice shot at winning, as Psystrike is a 2HKO on Blissey and Fire Blast is an OHKO on Skarmory, and Blissey won't get a Softboiled in as long as Mewtwo is staring her down. And even if Blissey lands a Toxic, she will be at too low health to be able to Softboiled. Skarmory will be OHKOed barring Sturdy, and if SR are up then Skarmory is completely DEAD.
But then again, Zekrom can just Bolt Strike them both, and it OHKOes. So sorry Mewtwo, you are second place.
Pokemon Showdown username is Serenity.