This is true, as several guides and characters confirm this:
"The Sith have waited millennia for the birth of one who is powerful enough to return them from hiding. Darth Sidious is that one—the Sith's revenge on the Jedi order for having nearly eradicated the practitioners of the dark side of the Force."
The Complete Visual Dictionary
"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
"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
"Meet Darth Sidious – the most powerful Sith Master who ever lived."
Darth Maul, Sith Apprentice
"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
"Yoda was a master at masking his emotions, but not even he could hide them from the greatest Sith Lord ever known."
Clone Wars: Wild Space
"Their clash transcended the personal; when new lightning blazed, it was not Palpatine burning Yoda with his hate, it was the Lord of all Sith scorching the Master of all Jedi into a smoldering huddle of clothing and green flesh."
Revenge of the Sith novelization
"Yoda went after Palpatine in the empty Senate chamber, but could not defeat the most powerful Sith Lord in history."
The New Essential Chronology