Honestly I think a Evil Mirror world of MHA where Big Good and Big Evil stays the same(AFO, Garaki, 1 to 8 OFA users) would be pretty interesting since a lot of the heroic side has some pretty terrifying and destructive quirks and are generally competent.
Which characters do you think would get terrifying and disturbing really fast?
So how this would go depends entirely on All for One's moral code in this mirror world. All for One is such a broken quirk that allows him to completely change the board at any time if he has the will to do so. Give him too strict a moral code and he could end up like Momo whose quirk has endless potential but is severely limited by her moral code and her limited creativity. With anything other than pure morality for All for One, the power of the other side doesn't matter much.
Granted, in a mirror world where All for One became a symbol of stability early on, it would have ended around the present in BNHA. Good All for One wouldn't have used someone to make a new body for himself. Even if he shared Garaki's quirk to live longer in this mirror timeline, he would have been near the end of his life. A society where it is known quirks can be removed and exchanged would have a very different conception of identity than canon, so it would fundamentally change the story.
But if you're asking who on the hero side would be the most terrifying as villains, some contenders are All Might, Hawks, Izuku, and Momo. All Might and Hawks are highly competent and live for their roles. In canon, they devoted everything to their roles as heroes. If they brought the same energy to being villains, it's questionable what would be left standing.
As previously stated, Momo's abilities are nerfed by her allegiance to the hero side. She is 1A's best tactician and her quirk has infinite possibilities. She could break the economy without even starting a fight if she really wanted to do so. There's also just so much more she could do with the fact that she can produce weapons from her body. Lady Nagant demonstrated how dangerous an opponent whose body can act as a weapon can be. And Momo could be a Swiss Army Knife Lady Nagant with the right motivation.
Izuku with or without One for All would be a dangerous villain. He is also an excellent strategist and he knows how to pick people apart from their quirks to their emotions. He has a lot of ambition and the potential motive to oppose hero society overall.
Just having a destructive quirk doesn't say much about how terrifying they would be as villains. Despite having powerful quirks, characters like Bakugou, Endeavor, or Mirko wouldn't be particularly dangerous to the system. At the end of the day, their goals were to be the strongest. Their villain equivalents are characters like Muscular and Rappa. Yes, they are extremely powerful, but they are so self absorbed and hotheaded that the damage they can cause is limited.
