vader; for obvious reason. Dude has become a cultural icon of evil and is the only Sith with any character development.
Dooku: because Christopher Lee. He has no character but Christopher Lee makes him awesome
Maul; boring, no motivation, no lines to make him interesting, no development, no actor to make him awesome with just the sound of his voice. the only thing cool about him is his design but it's not enough. Dooku has some character with his fight with Yoda but Maul has absolutely nothing