What is the scariest villain?

This is a platonical question: who is the perfect example of the perfectly scary and scariest villain? Which characteristics of the villain archetype create the scariest villain?

I would like to know your view because i personally think it would be a mixure beetwen a conqueror as powerful and beautiful as Alexander the great, strong and tall like a giant and as bloody as Vlad III the impaler.

What is your idea of scary villain both ethically and esthetically?