Who are the greatest Supervillians in all Fiction?
Hmmm this is a tough one.
To be unoriginal I would say the Joker. The man is bat crap insane. There are no limits, he spreads chaos and death like it's a game and there are no rules, not even his followers/henchmen are safe from his lunacy.
However I do kind of think Marge Simpson is a pretty good villain. Not a super one but a villain none the less. She knows what's going on and let's it happen, makes excuses and ultimately does nothing to stop her children from causing chaos and mischief.
Does the giant marshmellow monster in ghostbusters count?
Yes, sure it counts. In fact, I think that it would be nice to see more supervillains included that are not strictly from Marvel/DC comics. For instance, the LOTR, Harry Potter, Beowulf, Inheritance Cycle, ect. ect. universes are all welcome in this thread