"The Sith Emperor is the most powerful Force-user who has ever existed."
I can see your still raging after I Sidioused-out the Vitiate thread...
1. The quote is of TOR era, applying to all the Sith who come before him. How could it include all the Sith who came aftermath when at that time in history, they simply have not yet existed?
2. Even if you find some way to condrict statement 1, you have one accolade, I have 5, all from various sources (So I win):
"Ob-Wan and Yoda were the only Jedi left in a position to do something about the disaster. Yoda went after Palpatine in the empty Senate chamber, but could not defeat the most powerful Sith Lord in history."
-From The New Essential Chronology
"When Yoda crosses sabers with the movie's arch-villain, he doesn't launch into a pinwheeling display of acrobatics, as he did against Count Dooku in Episode II. Instead, Yoda faces the dark side's fury, channeled by the most powerful Sith Lord in history. "Rob Coleman wanted Yoda to feel the power of his enemy," says Wheless, "like a force he's never dealt with before."
-Insider #86: Yoda's Right Arm
"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
"It quickly became clear to Luke that this decrepit and seemingly defenseless old man was masterfully adept in the ways of the Dark Side of the Force. Indeed, as Vader had warned, the Emperor had become the Dark Side's most powerful expression."
-Dark Empire (Endnotes)
"Inside the spacious interior of the Galactic Senate chamber, Yoda challenged the Emperor. The two engaged in a spectacular duel—a contest between the most powerful practitioners of the Force’s light and dark sides."
-The Complete Star Wars Encyclopedia