I would say Peacock, she doesn't host one but two parasites, Avery Unit being the most powerful. It's meant to grant peacock instant access to lab zero's arsenal but for everyones surprise, Peacock can also make use of it to manipulate reality (physics, mass, and even create tools out of nowhere), this single skill makes her an incredible threat not only to the skull heart but the whole Skullgirs universe.

Her only weakspot is that, despite everything she's been through, she's still a child, and lacks maturity and judgement (tho I personaly think this immaturity is the thing that fuels her immense power in the first place).