but he did not learn everything he even admits it, in Episode 3.
He knew he could not defeat his master Darh Plagueis face to face, so sidious killed him in his sleep...Sidious admits his master was more powerful. Also to name a few sith lords to contend for the title (not all)
(Darth Bane does mention it, its mentioned a few times through out the Darth Bane trilogy, even looses faith in Zannah at one point when she uses assassins)
Darth Bane (arguably zannah cause she kills Bane but then again Bane was not in his Prime and was already dying)
id put them all above Sidious, also were slightly of the topic so sorry
Sidious is the most powerful sith to ever live. He could make wormholes that can destroy surfaces of planets and entire fleets. He is very powerful though I think he reaches this power in the novels.