Ranking Sidious' Accolades
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
RotJ Sidious: The Dark Side's most powerful expression period. The Supreme Master of the Dark Side of the Force. He had succeeded where all others had failed in taming it.
As Luke's father once said, during the time he served the greatest known wielder of the Dark Side of the Force, the Emperor: "The ability to destroy a planet - or even a whole system - is insignificant next to the power of the Force."
--Dark Empire endnotes
ANH Sidious: The greatest known wielder of the Dark Side of the Force.
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
ANH Sidious: The most powerful Sith ever (as per Vader).
With the galaxy now ripe for conquest, the Emperor has become the most powerful Sith Lord of all and a master of the Dark Side of the Force, ordering the extermination of the Jedi Order with the aid of his apprentice, the deadly Darth Vader.
--Legends Epic Collection: The Empire Volume 1
RotS Sidious: The most powerful Sith Lord of all.
Arguably the most powerful Force-user in general as of RotJ:
And what of the Emperor? He was mighty in the Force, perhaps the mightiest being who has ever lived.
--Galaxy Guide 5: Return of the Jedi
RotJ Sidious
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
RotS Sidious: The greatest [living?] practitioner of the Dark Side.
Beyond the vision of the Jedi Knights, somewhere within the darkness, the greatest master of evil ever to use Sith power bides his time. As his strength grows, his plans begin to shape the course of the galaxy, and his snares await the unsuspecting.
--The Complete Visual Dictionary
TCW Sidious: The most powerful dark-sider to have ever been Sith.
Meet Darth Sidious – the most powerful Sith Master who ever lived.
--Darth Maul, Sith Apprentice
TPM Sidious: The most powerful Sith Master (as per Maul?).
More affirmation that RotS Sidious was "the most powerful Sith Lord in history."
Yoda went after Palpatine in the empty Senate chamber, but could not defeat the most powerful Sith Lord in history.
--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.
--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
Emperor Zaarin? The idea isn't as ludicrous as it sounds. Demetrius Zaarin gambled everything on an audacious coup d'état and nearly killed the most powerful Sith Lord the galaxy has ever known.
--Insider #66: Who's Who: Imperial Grand Admirals
One more.
When the most powerful Jedi battled against the most powerful Sith, the two sides of the Force clashed in spectacular style. Grand Master Yoda took on Darth Sidious in the Senate building on Coruscant and proved that strength and power have nothing to do with size.
--Jedi Battles
RotS Sidious: The most powerful [living?] Sith.