Darth Ventress is a compelling choice! She has a complex character arc and her journey through the "Star Wars: The Clone Wars" series is fascinating. Her skills in combat and her struggle with her identity as a Sith and her past as a Nightsister make her a standout character.
If I had to choose a favorite Sith Lord, it might be Darth Revan for his deep backstory and the moral ambiguity of his character. His transformation from a Jedi to a Sith and back to a hero adds layers to his story. Who else do you like alongside Ventress?