I don't quite understand where this is comming from.We simply are expected to take at face value who is the most dangerous villain according to the story, but we aren't given self evident reasons to believe it.
Obviously All for One is the strongest and most dangerous villain.
He can basically own every single ability and thus has always an answer to someone he goes against.
Other than that, we aren't really told who is the strongest or most dangerous villain in this show.
The rest are just villains who have the criminal energy to be a villain. It doesn't matter how dangerous they are compared to someone else. It was never truely about the ability potential. It has always been about what the individual achieved with their ability and how he used it and what he wanted to do with it.
Stain for example killed a lot of heroes with his ability. Thus he is a dangerous individual because he uses his quirk to kill heroes/people.
That's it.
The same goes for Overhaul.
He is elusive and cunning and it was more about him staying in the "legal" area and the heroes had to find some dirt, so they could invade his hideout.
His ability wasn't what was dangerous, te technology he achieved by torturing the small girl is what made him a potential threat to all heroes and people in general.
And we've seen what that technology did. We have multiple heroes that are now without any powers. Including Lemillion.
I never felt like that the story told me "you have to fear this guy because he is super powerful" - aside from All for One of course.
The Villain group, in my eyes, isn't even actually truely powerful. As mentioned earlier Toga seems to be average at best and the whole group itself more like a "villain-training-group". The opposite of UA, so to say.