I think that the Joker makes the best villain. Why does the Joker make the best villain, you may ask? Well, like his counterpart, the Batman, the Joker has no super power or special ability, but unlike the Batman he has no cause to do what he does. Not only that, but his blood alone, is, as I would put it, "pure evil in itself," as the toxins that he discharges out of his lapel flower are actually made from his own blood. Furthermore, the Joker is immune to poison, suggesting that his blood is poison.
The exact way he actually managed to use his blood as a toxic weapon is unknown. However, I presume it involves black magic and witchcraft. Since his blood is, as I called it before, "pure evil in itself," it would make sense if his blood would serve as the darkest most powerful priming fuel for black magic rituals involving blood magic. Blood magic is a form of black magic that uses blood as a power source. Not all rituals require blood as a power source, however, if you were to add blood, although it would make the ritual have a greater effect, it would also most likely, depending on the ritual, put you in contact with evil and/or angry spirits, instead of peaceful or neutral spirits, and abusing black magic can end up withering away all good in you, and consuming you with corruption, which is exactly what we see in the Joker.
Overall, the Joker creates mayhem for the sake of chaos, making him the most dangerous, ruthless and psychopathic villain we have ever seen.
