Sidious is most powerful sith and luke:
“Yoda went after Palpatine in the empty Senate chamber, but could not defeat the most powerful Sith Lord in history.”(The New Essential Chronology, page 84 )
The Emperor had known it would be thus, of course [that the Rebel Alliance would be troublesome]; the resistance had not been a surprise to him. The Emperor was completely in concert with the dark side of the Force. He was the most powerful Sith who had ever existed.” (Death Star, page 76)
“Vader imagined the power that could be his if he crushed Palpatine and established his own rule over the Empire. But first, he would need his own apprentice. By himself, he could not hope to defeat the most powerful Sith Lord the galaxy had ever known.” (Vader: the Ultimate Guide, page 19)
"Palpatine has spent decades studying the most arcane and esoteric Jedi disciplines. It is believed that he has mastered nearly all the known powers, previously unknown powers, and devises new ones at his pleasure"-- The Dark Empire Sourcebook.
“[The Galactic Emperor] had succeeded where all others failed in taming the Dark Side. He would journey across the universe, spreading the shadow of his rule, blotting out the stars themselves, and taking his Dark Rule to other helpless galaxies.”-- The Dark Side Sourcebook.
”Beyond the vision of the Jedi Knights, somewhere within the darkness, the greatest master of evil ever to use Sith power bides his time.”-- The Complete Visual Dictionary.
"The Sith Order, in hiding for a millennium, had awaited the birth of one who was powerful enough to return the Order to prominence. Darth Sidious was the fulfillment of that prophecy, capable of exacting the Sith's revenge on the Jedi for having nearly eradicated the practitioners of the dark side of the Force." -- the Complete Star Wars Encyclopedia.
"Only Palpatine has been able to spread his darkness completely and totally over an entire galaxy. What has proven to be the lasting genius of Palpatine as Emperor is his devotion to collecting all the knowledge of the Dark Side that he can, as well as what Light Side information he can corrupt and preserve. He is no more altruistic than any other, but his newfound immortality has given him the patience that all before him have lacked."
The op said most powerful force user. All your sources say most powerful SITH. Palpatine Admitted that Plageius's apprentice could not ever hope to defeat him, he was too powerful, he WAS the apprentice... The only reason he killed him was cause he did it in his sleep