These are all fictitious characters. If you really want someone that is a real person who is absolutely to me the most horrific, twisted, magnetic killer - try Charles Manson! Not only what he has been involved with will scare you, but take a look at him - he needs NO Hollywood makeup - he is the epitome of evil incarnate.
Chucky's probably the scariest in my mind. I mean a homicidal, somehow alive doll...creepy! There's also the Leprechaun, but I think Chucky is worse.
It depends on your metric. If you're thinking in terms of 'most nights' sleep lost for the hero', I think Green Goblin is probably up there. But my vote for 'other' goes to Carnage, mostly from his original run.
I think GOD is the scariest villain ever. His followers have perpetrated more evil than all the of the creatures on that list combined. His image is a composite of the face of humanity and we are a creation that does not like itself and thus do not like our creator.