The larger question of "what is evil?" is, of course inextricably tied to its opposite, "what is good?" That's not entirely cut-and-dried either. I just finished Kierkegaard's Fear and Trembling, which argues that Abraham's greatness came from his willingness to commit an act of evil - murdering his own child - on the what Kierkegaard calls the "virtue of the absurd", the belief, essentially, that impossible things are possible through God. However, Abraham's recognition of this required him to suspend his finite sense of ethics and morality and act on a higher plain of faith, so that his actions, if subject to ethical standards, were morally indefensible. Abraham faith was such that he was certain that he was following God's will. Had he been wrong, or had he even been slightly unsure, he would have been a monster, not a hero. Thus it's easy for believers to admire his actions superficially, but difficult when you dig deeper and attempt to apply them.
Well, so much for a clear good/evil dichotomy, but Disney at least operates on a simpler level than Hegelian-influenced Christian existentialism. I don't think it's possible to establish any sort of objective taxonomy for ranking evil, so I'll try to approach it holistically with a healthy dose of my own prejudices to guide me. Also, this isn't comprehensive because there are a few movies I haven't seen or forgotten.
Laidy Tremaine (Cinderalla's Wicked Stepmother)
She's manipulative, petty, and willing to destroy her stepdaughter's life due to her own jealousy. Though her brand of evil is limited in scope, its intensity and depth are very real given the effort she puts into creating a private hell for Cinderalla.
A case study on the dangers of power and corruptibility. Her insularity and absolute authority (unchecked by her meek husband) lead her to execute anyone she views as challenging her. Though it's played for laughs in the movie and most of the would-be victims are non-human, it's really quite chilling with a lot of real-world parallels. Oh yes, she's a dangerous one.
Hook is more obsessed than purely evil, though kidnapping and conspiring to murder a child are hardly admirable. Still, Peter kind of has it coming, the little brat. I think the real villain of the story is Pan, the sociopathic petty tyrant.
She turns into a freaking dragon! Aaaaaaaaahhh!
She's an interesting case study on what it means to be evil, because while she's certainly a bit unhinged the objects of her fury are mostly dogs. Granted, they're talking dogs we can identify with, but the relationship is asymmetrical. To them she's a monster, to her they're a fashion item. I wonder whether Cruella was to some extent a product of her society. Would her dog fur coat have made her an outcast among most humans, or would it have been accepted? Whatever the case her thievery and mistreatment of friends and employees means her evil cred is legit.
I actually sympathize with him. He's just doing what a tiger does, and while he can be faulted for not trying to get to know Mowgli before condemning him, it's not like he has jungle-boy precedent to go on.
Selfish and naive, another abuser of power. Sent his own brother to die in a crusade, and seldom thinks twice about oppressing his people. He leaves much of the dirty work to the Sheriff, but he surely has knowledge of it and that makes him morally culpable. Perhaps some of his "evil" stems from an inability or unwillingness to acknowledge that he is out of his depth, rather than pure malevolence.
I'm grouping them together because they're very similar: both are willing to kidnap and abuse children for their own profit. Both view other people as a means toward accomplishing their own ends, and as such are quite terrible. Mcleach, in particular, is best seen as a metaphor on the limits of libertarian ideology.
By her own words she was banished for trying to usurp the throne, and that's her primary motivation throughout the movie. She imprisons dozens of mer-people indefinitely and attempts to murder several others. I do have to wonder, however, what the movie isn't telling us. Why aren't any other half-octopus mer-people shown? Is King Triton as tolerant as we're led to believe, or was Ursula cast out in some kind of broader purge? I'd like to know the whole story, but as the movie presents her she's plenty bad.
Ignorant, xenophobic, and aggressive, Gaston is the archetypical right-wing villain. He has no respect for women, he uses prison as a political weapon, and he foments war based on fear and incomplete intelligence. With his strong chin and deep baritone, he'd be an ideal Republican presidential candidate. Slogan: "That makes America the best. And don't we deserve the best?"
Another would-be usurper. Not that Jafar's very sympathetic, but you can sort of see where he's coming from when you look at the Sultan. Maybe it's just bias against short and round people, but the the guy doesn't project competence to me.
Murders his brother, sends his nephew to die, leads his people to near starvation, and lets enemies overrun the land. Pretty bad, right? From a lion's traditional perspective, absolutely. But for the Hyenas, it's pretty clear that they'd been marginalized by lions in spite of their superior numbers, and for them Scar represented a real chance at equality, freedom, and food. Like many revolutionaries Scar proved a poor administrator once in power, but shouldn't he be given some credit for giving the downtrodden reason to dream?
Genocidal, manipulative, and fanatical, Frollo uses religion as a tool to commit all sorts of heinous acts. Then there's his bizarre fascination with Esmerelda, his possible occult connections, and his cruel manipulation of Quasimodo. His brand of evil, which scapegoats minorities as the cause of moral decay while promoting violence to cleanse society is, is deeply fascistic, and that, more than anything else, makes him the most evil Disney villain overall in my view.
Most evil? Well, he does summon ancient monstrosities in order to literally overturn the existing order of heaven and hell. Judged purely by the extent of the damage he causes, Hades has to be right up there. How many died because of that, or would have if he succeeded? Still, he's so dang affable and entertaining that it's hard to dislike him.
This ethnic stereotype of a villain has pretty clear motivations: plunder and pillage China and overthrow its government. Once again, though, it seems as though we only get half the story. As he tells it China goaded his invasion by building its wall, but what had happened previously to stoke such animosity between them and the Huns? Perhaps the Huns are primarily to blame, but history tells us that when it comes to war the line between good and bad gets very blurry.
So that's my take. Frollo's the worst, Gaston's pretty bad (though he gets the best song), and Maleficent, you know, turns into a dragon. If anyone has thoughts the matter, or on the nature of good and evil generally, comment away!