Depends... darth sidious was very powerful with the force and was the only one to actually achieve the sith goal - control the galaxy. Darth sidious was very force sensitive. but sidious was ruling in a time when there wasn't really any jedi around for him to face
1. Darth Vitiate was stronger. Darth Vitiate was able to demolish full planets and absorb everyones souls. His force powers were far beyond sidious.
2. Darth Nihilus - very very strong, he became an entity, a walking soul and could also destroy whole planets better than darth Vitiate. Vitiate needed other sith lords to help him, Nihilus could do it on his own. Also he can aborb energy from jedi and other sith, simoly making him stronger. Very powerful
3. Exar kuan, it took all the jedi to beat him. And even then he survived for another 4000 years after death. He was almost too powerful.
4. Darth Raven - he mastered the dark and light side
5. Darth vader - would of been the most powerful jedi to ever exist if he reached his full potential. That would of been amazing. He also was never a sith lord, he is considered a dark jedi. He could never be able to destory sidious as one shock from the electricity would kill vader.
Check them out... most of these very powerful sith lords are from the old rebublic with ancient sith teachings
On a somewhat related note, isn't kylo Ren a joke? Lol he acts like Darth Vader 2.0, but gets his ass beat by a girl that had barely even used a light saber, and chewie shot him in the leg. Like what a rookie move. What a little bitch right? Lol
You forgot about Boba Fett and General Grievous, but I voted for Darth Sidious, he was the most powerful, no doubt... and Jabba The Hutt is too slow to fight any of them, but he is kinda funny and cool character :D
I think as we follow his character arc it may be Kylo Ren. Sure he may have seemed like a cry baby in this movie, but he is one of the more powerful force wielders in the new canon after Disney obliterated most of the old canon
Do you mean powerful as in influence, or powerful as in fighting ability? Straight power we'd go with one of the Sith from the EU, before it was destroyed, like Naga Sadow or Exar Kun, who basically required armies to be defeated. In influence over the Galaxy, probably Sidious.