although vader defines starwars I think sidious, the freaking emperor lets not forget it, is the absolute defenition of evil.
he doesn't beat around the bush, he manipulates, dominates and kills to get what he wants. He staged the largest war ever and tricked every jedi including windu and yoda for many years while right under their noses.
This role is only magnified by the mysterious cloak, face shading hood and slide hunch he is sporting. His every move, his every grimace, his very siloutte defines the total and complete incarnation of evil. And vader is his avatar, awesome, but a servant non the less.
(remember all his plans, everything, worked perfectly for 6 movies for all but give or take 15 minutes)