Hard choice.
Frollo probably takes the cake on that one.
Mother Gothel is a close second though.
Gaston and Scar
Chernabog also terrified me as a child and I still can't watch Fantasia.

Scar is the obvious choice...

Gaston from Beauty and the Beast in another. He wanted to kill the Beast just because Belle wouldn't marry him D:

does the extended universes of StarWars and Marvel count?

cause i'm pretty sure there's a LOT of evil characters though out those 2 newly aquired franchisees. 

 Frollo, He is absolutely bonkers, Scar, he manipulated and killed his own brother, Mother Gothel, who is so desperate to live forever she would kidnap children and kill people.

Micheal Eisner!!!!  He's the one that said people don't like traditional animation any more and had them close the 2D department!!!  (And then John Lasseter reopened it again for two movies before shutting it down again, the tease!!)

Also 'The Coatchman' from Pinnochio.  Turning kids into donkeys and then selling them to salt mines (psst and he gets away with it!! At the end of the movie he's still doing it!!!!)

I'm surprised Stromboli hasn't been mentioned yet. People must be forgetting how fucking terrifying he was. 

I have a hard time answering this because my favorite Disney villains weren't actually created by Disney, and the genius is owed to their origional book authors (like with Claude Frollo.)
Of their origional works of have to say Mother Gothel, who was both very origional and creepily familiar in the 'manipulated children with years of isolation and brainwashing' kinda way. Plus I really like her villain song. 

I would say Hades, Scar, and Zira.

The Devil from Fantasia. No explanation needed

Gotta go with the queen in Snow White - she not only orders her step-daughter killed, but demands her heart as proof, all because Snow White is deemed hotter by a magic mirror?  That's cold-blooded.
And for someone so obsessed with her own beauty, she then ups the crazy by turning herself into a hideous old crone, just so she can feed Snow White some poison.

Is it too soon to say Darth Vader?

The hunter from Bambi.

Without doubt, Scar. Not only does he kill his own brother he also manipulates Simba into thinking he killed his father and then sends him into exile. Sounds like the perfect description of villainy in my opinion.

But Maleficent comes a close second. 

Ratigan. Well he is voiced by Vincent Price after all.

"Complete Monster"?

A lot of people mention Scar but they don't realize is what he really did. After exiling Simba, the three Hyenas were most likely rewarded with eating Mufasa. :I

Hmmm I was going to say Scar- he killed his own brother!! But yeah I would definitely have to agree that Frollo could be the most evil

There is nothing more creepy than Claude Frollo: A crazy, murderous catholic fanatic with lustfull thoughts about a girl who he wants dead. When I was little, I was terrified about this villain

My childhood favorite was always Maleficent because of her unique aspect, but I'd have to say Ursula she was indeed a monster., but like so many others including Scar, Jafar and Hades and Lady Tremain, she was just hungry for political power. 

Hans! No doubt about it. All his character screams evil. 

Scar in the Lion King is as evil as it gets, atleast out of the Disney movies anyway.