Palpatine, also known as Darth Sidious, was a Force-sensitive Human male who served as the last Supreme Chancellor of the Galactic Republic and the first Emperor of the Galactic Empire. A Dark Lord of the Sith who followed the Rule of Two, an ancient tenet of the Order of the Sith Lords, he was the most powerful Sith Lord in galactic history. His entire life as a Sith was the culmination of a thousand-year plan to overthrow the Republic and the Jedi Order from within.
it is said opening lines of wiki page first
most powerful here can be understood,as holding the most political power. I highly doubt he is the most powerful force user in history.The EU in the events after the movies with the worm hole and all that clone bodies nonsense is crap anyways.Best case scenario he is the most powerful in the Rule of Two sith,even here i am not sure he is more powerful than Plagueis,because he didn't kill him in a combat as Bane commanded but while he was drunk and asleep.
But yes he is the most powerful being in his time,in an age where the jedi are a pathetic complacent bunch.
Naga Sadow and Darth Nox(even without the sith spirits) would eat him for breakfast,not to mention Tulak Hord.