How about Grand Moff Tarkin? In A New Hope, Vader is just following orders, and it seems (to everyone, the movie goers and the characters) that Tarkin is the real ring-leader here. Hell, he's the one who makes the call to blow up Alderaan after getting the (supposed) location of the rebel base, just for the hell of it. The man single-handedly wipes a planet from existence for shits and giggles (and to torture Leia more). Honestly, if that doesn't put him in the running, I don't know what does. Also dem cheekbones.
Good point. I feel Tarkin does have the real power but he just doesn't have that X factor, the intimidation that Vader has. Of course going along your lines of argument the Emperor could also be considered the best villain, since he also was directing Vader. I don't have a problem with that actually, he's one sinister, creepy looking mofo.