It is difficult to say for certain who would win in a hypothetical battle between Obi-Wan Kenobi and Emperor Palpatine. Honestly? I think the Jedi would've won that battle and the Sith would've been defeated and Anakin might have even possibly have been redeemed back to the light side.
Originally Answered: Could Obi Wan defeat Palpatine in a duel? No.. He can not.. And BOTH Yoda AND OBI WAN himself SAID SO both in the movie and the book Revenge of the Sith.
If Obi-Wan had faced Sidious, not only would he have died, but all hope would have also been lost in ever defeating the Sith and bringing balance to the Force.
During his fight with Palpatine, he equally matched the Sith Lord and came very close to beating him. It was only when deflecting lightning that Yoda lost his footing and fell down the Senate Chamber, taking him out of the fight. But if Obi-Wan was there to assist, it could've been all Yoda needed to defeat Palpatine.
Palpatine absolutely did NOT fear Kenobi as a physical threat to himself. Could he have beaten Obi-Wan in a straight lightsaber duel? Possibly. Let's compare their forms.
Palpatine admits to Darth Vader that the Jedi he actually feared most was none other than Jocasta Nu.
As powerful as Darth Sidious may have been, it's easy to forget he actually seemed afraid of Master Yoda.
This suggests that Mace Windu would still fall to Obi-Wan. However, the Powers of the Jedi Sourcebook says that Mace Windu is second only to Yoda as a duelist. The book does not discuss how Obi-Wan ranks compared to Windu, but at best it's a draw when it comes to their skills.
Star Wars Confirms Darth Vader Was ALWAYS Obsolete, Thanks to Palpatine. While Emperor Palpatine seemingly did everything in his power to keep Anakin alive, he still made sure to nerf a major part of Darth Vader.
Darth Sidious was able to cheat his upcoming death by transferring his spirit into a clone body while falling into the reactor core of the second Death Star. He later sought to prolong his existence by futilely manipulating his grandchild Rey into killing him so he could live further by possessing her body.
In truth, the Sith normally wielded enough powers to fight without a lightsaber, but every apprentice still learned to use one as part of his or her training. Sidious himself felt that he and his peers had outgrown the use of lightsabers, and only continued carrying them to mock the Jedi.
If Qui-Gon had survived his fight against Darth Maul, he would have sensed Palpatine's evil, and Anakin wouldn't have become Darth Vader. Thus, while Qui-Gon was less powerful than Yoda, Palpatine feared him more.
Aware that the Jedi Order could be destroyed, Windu sought to prevent this outcome by leading a team of fellow Jedi Masters, Kit Fisto, Saesee Tiin, and Agen Kolar, who were among the finest warriors in the Order, to arrest Palpatine, leaving Skywalker behind.
There has been no Sith, as powerful, meticulous, nor as evil as the Emperor himself, Darth Sidious. As previously mentioned, Sidious trained in the Dark Side by Darth Plagueis before Sidious killed Plagueis in his sleep, making Sidious a Sith Master.
For Now - Luke Skywalker. In Legends, Luke Skywalker was the undisputed most powerful Jedi to have ever lived in our knowledge, surpassing Yoda.
His use of vapad would make the fight play out similair to windu vs palpatine. He would use the seething rage pouring out of maul against him, and as mauls force power was so much weaker than windu's it would be an undisputed win for windu.
Obi Wan is one of the most powerful Jedi to have ever lived. He is the best defensive duelist in galactic history according to Mace Windu and numerous factbooks. He had a low sensitivity to the force but mastered it to his fullest potential worshiping the force as opposed to bending it to his will.
Arguably, Qui-Gon is wiser in some ways, given that he knows how to preserve his consciousness in death. Yoda is wise, but sometimes flawed, given that some of his views align with the problematic thinking of the Jedi of this era.
The reason why Qui-Gon Jinn is the most powerful Jedi in the galaxy is because he had seen everything that would unfold throughout the Skywalker Saga in a vision of the future, sacrificing himself to ensure the fulfilling of the Chosen One prophecy.