The scariest was most definitely Death Sword. Not only does he look like death reincarnated, but the way he appears is scary. First you have to realize that you need your senses. Then you use them and see this figure and you freak out. Then he becomes visible and you freak out even more. Not to mention the whole evil ghost thing he's got going on. Yeah, dude is scary.
Although, I think I myself was scared more by fyrus. Standing there like some beast sleeping. Then he wakes up and roars in your face. He's more intimidating, but Death Sword is scary.
Twilit Igniter Fyrus! just like Zelway mentioned. It is something really bizarre when a body is in chains (enchained?) and it made me feel sick when I faced the boss in question. Insanity is something that can scare me (but actually also make me laugh), I guess it depends on the "form".