Palps

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


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 the Sith finally emerged from a thousand years of watching and waiting, they numbered—in accordance with the tradition set down by Darth Bane—only two. The most powerful of these was Darth Sidious, an ice-cold, diabolically calculating genius equipped with the strength of the dark side of the Force, as well as an enormous wealth of Sith artifacts, equipment, and knowledge.

- The Dark Side Sourcebook


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


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