I think it is between Anakin and Darth Sidious becasue Anakin destryed the jedi, was the chosen one and defeated an extreamly powerful sithlord. Not to mention he kicked ol obi wans ass! But Sidious kept himself secret when around the other masters (even Yoda :O)
and ruled the galaxy. He brought the downfull of Windu, corrupted Anakin, and defeated Yoda in a dual. He did alot more things as did Anakin (remeber Ani was the greatest pilot in the galaxy though thats nothing to do with being a sith) but I think:
Sidious 1: First place for his messed up face
Vader 2: A truly awsome fighter, force user.
Revan 3: Revan was epic, put it that way.
Exar Kun 4: Very powerful and dangerous force user.
Bane 5: I don't know anything about him
PS: There would be others like Traya, Nihlious, Sion, Malak, the person out the force unleashed whos name I forgot, Duku, Maul ect but they wern't on the list