Among Sith:-
Sith are generally intimidating and dangerous individuals. However, following are stuff of nightmares:
Emperor Vitiate. He might grant you immortality only to use you as a feeding apparatus. He might reduce you in to a puppet of his will, taking away your freedom to make decisions. If in bad mood, he may just destroy your mind and leave you in a vegetative state or dead. If you somehow manage to cope with telepathic subjugation, you are going to be cooked by his powers.
Darth Nihilus. He is only interested in feeding on you, alive or dead.
Dread Masters. A Dread Master might reduce you in to a puppet of his/her will or drive you insane or unleash monstrosities on you that may devour you.
Among Jedi:-
Revan. He is unpredictable and won't hesitate to eliminate you if you stand in his way.