I'm not sure if this has been stated, but I believe that the most powerful Sith would've been Darth Nihilus, Lord of Hunger.
Yes, his boss fight was pretty laughably easy. But, in lore he has the ability to destroy entire planets just because of his hunger for the force. The boss fight wasn't difficult because The Exile was such a wound in the force that his, eh, appetite, wasn't as appealing as it was beforehand.
In terms of raw power, Nihilus wins hands down.
"He studied the ancient ruins on the Sith mausoleum world of Korriban. He unlocked secrets of the Force from a captured Jedi Holocron. The dark side energies flowing through Palpatine's body were so intense, that they ravaged his mortal frame. The very source of Palpatine's strength was killing him.
To counter the dark side's consumption, Palpatine turned to a bizarre combination of technology and Sith magic. Palpatine used Spaarti cloning cylinders to create a store of younger bodies, and employed an ancient Sith technique to transfer his consciousness into a waiting clone. Thus, Palpatine could avoid death indefinitely -- as long as his supply of clones remained intact. He would change his form again and again, prolonging his life. Palpatine constructed a secret throne-world deep within the galaxy's core, on a shadowy world called Byss. Here, he kept his clones safe, protected by a loyal cadre of Dark Side Adepts. "
"In true Sith tradition, Palpatine murdered his Master upon achieving the skill and ability to do so."
"Inside the spacious interior of the Galactic Senate chambers, Yoda challenged the Emperor. The two entered into a spectacular duel -- a contest between the most powerful practitioners of the Force's light and dark sides. The Emperor proved too powerful to defeat. Though Yoda held his own for much of the duel, in the end, the Sith bested him. He realized that directly confronting the Sith would be doomed to failure. Defeated, Yoda slinked away into the shadows of the Senate chamber's cavernous depths, leaping into a waiting getaway speeder piloted by Bail Organa."
As Juno rescued the Senators, Starkiller confronted the Sith Lords who had been manipulating him for years. Lightsabers clashed inside the Emperor's observation dome, but Starkiller was ultimately no match for the power of Darth Sidious. Bombarded by Force lightning, Starkiller did not fight back but instead unleashed all the power of the Force within him, causing a tremendous blast that shattered the Emperor's tower and caused enough of a distraction to allow Eclipse and the Rebel Senators to escape. - Complete Star Wars Encyclopedia
“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."
*Force Storms Wormholes that can destroy starfleets
* Force-based subjugation of ~20 billion sentients and the simultaneous leeching of their life energy
* Corrupting an entire planet, transforming it from a neutral territory to "one of the most powerful dark side sites in the galaxy"
* Blunting the Force sensitivity of ~10,000 collective Jedi for over a decade
* Slaughtering trained Force Jedi with a single bursts of Force energy on one's death bed.
* Disintegrating highly trained Sith acolytes with a single gout of Sith lightning.
* Destroying ~50 armored stormtroopers with a single gout of Sith lightning
* Shrugging off a Force-enhanced explosion from the energy of a Force-user powerful enough to manipulate a Star Destroyer.
* Manipulating the minds of seasoned thousands (perhaps millions) of fleet officers and members.
* Mindwipe the people of Coruscant to make them forget the burial of a star destroyer.
Sidious was stronger than Nihilus.
P.S. His force wormholes were said to be able to destroy planets as well.