Ah the age old question: "Who's the most powerful super-character in your universe?"
Personally, I've always liked the trope that the sidekick is actually more powerful than the main character. The limitation being that they are so powerful that their ability is either near impossible to control or overkill in most situations.

So, if we're talking about raw physical impact, no one in the #MistressDomination universe can top Penelope (phrasing lol). Although her power is in its infancy in The Rise of Princess Prude it quickly grows to show true glimmers of it's absolute terror...and has yet to reach it's full potential. In theory, with enough denial, enough of a psychological break, Penelope could unmake existence itself. She could deny everything and have it pop like a soap bubble in the dreams of a Lovecraftian elder thing. Let's hope Kate can keep her sane.

That said, I don't necessarily view that as the most powerful ability. Call it an aspie's bias or a futzing about with with definitions but I believe my villain in The World Is Naughty Enough takes the cake for most powerful. Now, to be clear, there are many new powers in this third book but the one I'd crown is the power of suggestion. Not mind control, not domination, just the ability to slip a subtle thought into your mind that seems so reasonable that you accept it without question. Ya, she's no one to worry about. Huh, you're right, I didn't get a good look at her face. We should retreat, this isn't going well.

The reason I would crown something so minor is that when done well you have no idea she's doing it. The dominos are tumbling in your mind, you're giving her everything she wants, and all the while you believe that it's you're idea. It doesn't strike you that there's a problem until it's already too late.
Sure, other villains can make you gouge your own eyes out, or crush you like a bug, but this new one...she'll make you agree with her.