[The Might]

I'd say Sauron and Gandalf.
As Gandalf says in "The White Rider" chapter I believe, he is dangerous in his own way, and nothing in ME is as dangerous as he is, unless you would stand before Sauron himself.
He isn't scary because he chooses not to be, but he could potentially be.
So from those ho do choose to be scary, I'd nominate the Nazgul.