Marka Ragnos is the most powerful Sith. I don't know who the most powerful Jedi is. Probably NJO Luke.
And as for Sidious.
1. Han Solo shot him in the back, after Sidious picked off the Jedi Knights. And need I remind you he was DYING at the time? Han just helped him along. Still. He must be pretty darn powerful if he was on his deathbed and wiped out Luke and a large contingent of Jedi Knights. Brand was the Jedi who killed him. And he died in the process.
2. Sidious needs to learn not to keep his back on certain people. Lol. Vader was cripped. But he was also in a mechanical suit that multiplied his strength to be double what a man's is. Effectively, Deus, he could 'bench press' any human you'd care to name down a reactor shaft. And, Vader died in the process.
So yes. By that logic I could say Dooku couldn't beat a three foot tall muppet and that Mace got killed by a seventy year old man. But we all know there's more to it than that. 😛
as most powerful being, exar kun all the way. He's knowledge of the dark side is unsurpassed. He read almost all of the sith scrolls and achieved almost supreme power. His failing (like every other bloody sith) is one of arogance and his intellegence. The wisest being is yoda hands down. The smartest (yes there is a difference between smart and wise) I give to Plageous. Just a question really how can people vote for NJO Luke when Anniken Solo was obviously more powerful. Thats just my opinion and open to debate.
Though many people will hate me for this I must speak my mind. Lucas stop being stupid. According to Lucas himself Darth Vader was the most powerful thing the Universe ever saw, yet he okays all these novels were people become quit oviously more powerful than him.