You're all wrong, the most powerful Sith would obviously be King Adas. The one true emperor of us all! No but really he was the most powerful and probably rules the Chaos Realm as of now, he was the one and only Sith'ari. Darth Banes dreadful existence was nothing more than a Shadow to King Adas. Nihilis didn't hold a candle to King Adas, sure he was known as some super powerful sith that was a void in the force because he got hit by a superweapon and survived; however, he was never declared a god of his people, he was also a spec of dust compared to King Adas.
For the most powerful Jedi I would have to give it to Mace Windu, he was Grand Master of the order before Yoda and then Master of the Order after Yoda because he felt that he was not fit (humble of a Jedi) He was also the only Jedi to be able to Master Vaapad in a correct manner. Also as said in my previous post he basically gets decimated by Mace Windu, even if people say he wanted Anakin to see the Jedi as evil. However, if you will not accept that i'd say Master Ooroo who happens to be Odan-Urr (the writer/revisionist) of the Jedi Code. Yes I understand he was just a brain, but probably one of the most intelligent and wise Jedi that lived, and to Jedi that is power.