For me, it has got to be Maleficent. She is the most awesome and the most terrifying. All the other Disney villains are good but Maleficent is the one thing that the others ain't. Pure Evil.
Most Disney villains are evil because they are after something whether its wealth, power or a certain person of their desire. Despite wanting the princess dead, Maleficent doesn't need any of these that the other villains are after. She has a huge castle in the mountains, demonic minions at her command (despite being a bit stupid, they can still get the job done) and also has all the powers of Hell itself. For a villain, Maleficent is one person you don't want to cross paths with. She is basically like the Terminator. She can't be bargained with. She can't be reasoned with. She doesn't feel pitty, remorse or fear and will not stop until you are killed by her powerful curses.