Here is a list and reasons.
1st Darth Bane- He was freaking powerful and had knowledge from at least 4 legendary sith lords, including Revan.
2nd Darth Plagueis- He could create life, and Sidious had to resort to killing him in his sleep instead of fighting him.
3rd Darth Sidious- He was more powerful than Yoda.
4th Darth Vader- He was 80% as powerful as Sidious.