By far Vader was the most powerful sith. He killed all the Jedi but two. He killed the Emporer which put him at number 1. Luke was a wus and would not fight him because he knew he could not win. Yoda could not kill the Emporer nor Dukoo. Vader beat Dukoo with ease on his was to being a sith. Mace Windu beat the emporer and places him at number 1 since Yoda nor the wus Luke could not do it.