Fear is a curious thing. None of the Sith (or Dark Jedi) really frighten me.
Y'see, for me, scary means "sinister," not "monstrous." I'm not scared by roaring and rippling muscles, I'm intimidated when someone's voice gets very slow and quiet.
The only time when yelling actually got a reaction out of me is on Star Trek, when Riker starts bellowing orders, and even then, it's more of a "snap to attention" than a "cringe in fear" reaction.
The only Sith/Dark Jedi I'm actually scared of is Palpatine, and that's just 'cause he's smart. Maul never struck me as being particularly bright, and I could probably out-think Vader or Jerec (or any of Jerec's minions).
The velociraptors in Jurassic Park are more frightening than the T-rex because, as big and powerful as the T-rex is, the raptors are nearly as intelligent as humans; possibly moreso. It's that element of uncertainty that gets me, the thought that the monster might actually be plotting, and could outsmart me.
"The Beasts know much that we do not." -Ancient Jedi proverb