Maybe Polaris, she's kind of going down hill, mental health wise. Though Pietro may be able to take her out, but he'd never be able to strike Wanda, so she could incapacitate her brother with a hex and win.
I really want to see them explore Pietro some more. His own series was great, and he has power hardly glimpsed at just waiting to be explored. The son of Magneto should have more than just weak superspeed, and he does have much more.
It is, I think it's a great power and I like Pietro, but as the son of Magneto, doesn't that seem like a downgrade? "I can run fast, while my dad can manipulate the electro-magnetic spectrum!"
He could take both of them out before they even knew that they were supposed to fight. Sure, they're powerful, but they don't have a chance of using their powers if both of their necks are snapped, or if they're both just knocked out before they can do anything.
And I do believe he has gotten a speed boost lately, now able to exceed 300 mph. That makes him the fastest mutant there is.
And since neither of them are invulnerable to physical attacks, he takes this battle before it begins.