how is that even a serious question? who and where does it say sidious was the most powerful sith? that notion is laughable and amateur. if you have done any amount of serious research you would know that sidious was extremely weak and a tratior to the sith ideals. also not to mention that the level of lightsaber mastery far outstripped these modern day jedi/sith. so to answer this question malgus would crush him like a bug with the force and carve sidious up like thanksgiving turkey. its no contest at all
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