A Force lightning ability, Palpatine has been in contention alongside the likes of Darth Vader/Anakin Skywalker, Luke Skywalker, and Yoda.
Over the course of Palpatine’s further four movie appearances after the original trilogy, alongside TV shows and various other tie-in materials, the Emperor’s powers have been greatly expanded. The prequels showed just how skilled he truly was, with Yoda fleeing from his duel with Palpatine in Star Wars Episode III - Revenge of the Sith, as well as his machinations leading to the destruction of the entire Jedi Order. Although he was ultimately defeated by Rey and the Jedi in Star Wars: The Rise of Skywalker, Palpatine’s legacy is one of immense knowledge and power, and perhaps as the greatest for the galaxy has ever faced.
That has certainly been one of Darth Sidious’ aims. Palpatine, even more so than the Sith Lords who came before him, has quested for unmatched strength and eternal life. In a way, he achieved both, tapping into ancient Force powers to ensure he returned even after his apparent death in Return of the Jedi. No one learned more of the dark side of the Force than Palpatine, and he himself believes he’s the strongest Sith to have ever lived. In the book Secrets of the Sith, written as though from his in-universe perspective, he makes this claim directly:
"None in this galaxy has ever proved my equal in the power of the dark side."
Of course, the judgement of Palpatine being the strongest Sith ever in Star Wars is a little clouded given the author’s self-bias. Arrogance and hubris have brought down many a villain in a galaxy far, far away, and Palpatine was no different. And yet, there is value to his claims. Although the long lineage of the Sith means it’s hard to be certain, Palpatine seemingly possessed more dark side knowledge than anyone else. Not only did he possess Force lightning stronger than any other, but he was capable of turning it into Force storms across the galaxy. In the Star Wars prequels, Sidious was able to masquerade among the Jedi as Palpatine, hiding his true nature even from Grandmaster Yoda thanks to his ability in Force Concealment, masking his presence in the Force. Finally, in Star Wars: The Rise of Skywalker, he uses Force Drain to take the Force energy from Rey and Kylo Ren and make himself even stronger. All of these are rare, and many ancient, dark side Force powers that few know about, and even fewer still could actually pull off, let alone with the skill Palpatine does.
If the main target of the most powerful dark side in history.
The bigger question is where Palpatine fits in the overall pantheon of Force s. In the same book, he acknowledges the power of the Skywalkers but also states his belief of being more powerful still, with Anakin second. Vader may not have matched Palpatine as a Sith Lord, in part because he was hindered by the injuries sustained in his duel with Obi-Wan Kenobi. On the other hand, Anakin was the Chosen One, a manifestation of the Force’s will with an unprecedented midi-chlororian count, who brought balance to the Force; his connection to the Force and the potential of his powers is far greater than any other, but it's arguable he never truly achieved it. His son, Luke, possessed that same Force connection. In Star Wars Legends, Luke's powers greatly sured anyone else; in canon, he has achieved his own astonishing feats and certainly has a claim, but not to quite the same level of power as in the EU. Likewise, Yoda knew more of the Force than any Jedi before or since, but still couldn't stop Palpatine's rise. There are many factors to those debates, but what's clear is Palpatine at least came close to truly achieving unlimited power.
