Both are bad but I think that Scar was bad too. He killed Mufasa and tried to kill Simba too, though in the end he wined way too much still pretty bad.
In my educated and well-considered opinion (I'm kind of a Disney fanboy), the best of them all would have to be Maleficent, from Sleeping Beauty. She holds herself to a high standard, has very fastidious conduct with others, and moves very smoothly and poetically. However, offsetting her quiet sophisticated evil is the spiteful, high-powered, and utterly destructive evil we love to see from our villains. Maleficent, the self-proclaimed "mistress of all evil," and rightfully so.