Nowhere King/The General/Elk and Emperor Belos are some of my favorite villains and are very dangerous

The one who knows they’re evil but believe their cause is just - See Magneto, Albert Simon or Zaheer.

The ones that think they're good. Mainly because they can manipulate you & even deceive you if you are not careful.

Card-Carrying-villains are more dangerous as they will have less guilt and are more prepared to commit heinous acts as they do not lie to themselves.

Villains who know they're evil, unless the villain who think they are good is either really delusional, stupid (mostly due to poor writing) or naive, they mostly have standards and moral boundaries. This is why PE many times don't care wether they are doing good or bad at all, they are smart but don't care about their own wrongdoings, wether a good guy villain who doesn't have standards should be able to see what it does is wrong because his goals are "supposedly" good.

Compare Owlman to Thanos. Owlman understands the weight of his actions but doesn't care, he is not delusional to pretend he is a good man, his goals matters but is not good and he's fully aware. Thanos on the other hand is idiotic, he thinks he is saving others but committing omnicide when he could do something else that actually helps and he actually has a soft spot to himself. Owlman is smart because he understand the weight of his actions, Thanos deludes himself with them thinking he's good.

Owlman's crime would have been much greater, given he was going to kill every Earth in existence, which, mixed with what we know of there being multiple earths, would have killed more people than even Thanos's snap in a single Universe.

I think villains who think they're good are more dangerous because it means they see anything they do as justified. If they know they're evil, then they at least aren't delusional and they could still have lines they won't cross.