I would have answered Darth Vader, but then Lucas made the Episode I-III trilogy and I found out the most frightening villain of all time was nothing more than a whiney, easily manipulated brat, father of whiney Luke (okay, we knew that part, but we didn't have to have a "like father-like son" thing, did we?).
That totally ruined the franchise for me.
Oh, and Han shot first!!
Well, there's always Ming the Merciless...
Ming the Merciless - Wikipedia, the free encyclopedia