If you're asking about "Worst" Disney villain as meaning least effective, or worst-made Disney villain, I'd pick Edgar (the butler) from The Aristocats. Just a clumsy coward with no charismatic or respectable qualities found in effective, timeless villains.
If you're meaning worst villain as in the most evil, then by far it's Maleficent. She is, afterall, the mistress of all evil. (And to go THAT insane over not being invited to a ceremony??)
Interesting sidenote. Out of all the Disney animated films from Snow White to Tangled, the only villain to ever win in the end was the Headless Horseman (Brom Bones) from The Adventures Of Ichabod & Mr. Toad. Maybe not the most evil, but definitely the most effective considering he ran off the hero & got the girl. Not the usual ending to most Disney films.