Originally Answered: What Jedi did Darth Sidious fear the most? Luke Skywalker. Sidious had a vision that Luke could destroy the Sith, and he was concerned. None of the prequel trilogy Jedi frightened him.
Palpatine admits to Darth Vader that the Jedi he actually feared most was none other than Jocasta Nu. "Who?" I hear a lot of you of ask. Well, I'm sure most will recall hearing the name at some point. Now that you remember her, let's actually explain who she was.
Emperor Palpatine admitted to fearing Luke Skywalker and even said that if he had him by his side instead of Kylo Ren, he would have won the galaxy.
In Disney Canon: Galius Rax. Rax was a boy that Palpatine met from Jakku. Rax became a trusted adviser to Palaptine, and Palpatine entrusted Rax with his contingency plan.
Mace Windu failed in his mission to arrest Palpatine in Star Wars: Episode III - Revenge of the Sith, but Yoda may well have fared better. As powerful as Darth Sidious may have been, it's easy to forget he actually seemed afraid of Master Yoda.
In the novelization of Revenge of the Sith, we see that Palpatine feels genuine fear during this battle. Not only this but when asked about the fight George Lucas himself has said that Palpatine WAS overcome. This is due to mainly to Mace's fighting style, reflecting the power of Palpatine back at him.
When Qui-Gon Jinn showed up on Naboo and threatened to throw a bantha-sized wrench into his plot, Palpatine had to act fast. He dispatched Darth Maul to get rid of the Jedi Master once and for all. Therefore, just like Yoda said, Palpatine's fear led to evil deeds, death and the rise of the dark side.
Star Wars: The Rise of Skywalker hints at Palpatine's real purpose for Darth Vader. It's an open secret that Palpatine was disappointed with Darth Vader; he thought he was getting the Chosen One, a force of nature who could not be beaten, and yet Darth Vader was defeated the moment he crossed blades with an adult Jedi.
In non-canon material it is said that Palpatine had a number of concubines and never took a wife in fear that he would produce a legitimate heir. One of these non-canon concubines was Roganda Ismaren, a former Jedi who survived Order 66, and lived for a time at Palpatine's side.
Even after his brutal scorching in the fires of Mustafar, sand remained the only natural phenomenon that Vader feared. To him, sand was a reminder that he'd failed to save his mother or Padme from death.
8 WEAKNESS: Palpatine Is Inflexible & Relatively Shortsighted. One of Palpatine's greatest problems is that he cannot easily adapt to changes in the plan. This was proven during Luke Skywalker's stay aboard the Death Star, where he fully expected the young man to cut down his father.
Darth Vader is the rockstar of Star Wars baddies, ever since he first appeared in A New Hope. He represents the power of the dark side for generations of fans, but his legacy continues on in the Star Wars galaxy as well.
The reason Vader never successfully overthrew the Emperor wasn't from lack of ambition, but more from a lack of allies. Even in Revenge Of The Sith Anakin planned to surpass Palpatine by telling Padme that they could overthrow the Chancellor and rule the galaxy together.
Vader relies on the complex life support system of his suit to survive and it happens to be very vulnerable to electrical discharges. So, if Vader ever had unleashed Force lightning, there's a good chance it would've caused his suit to short circuit, thus, killing him.
The short answer is yes, without a doubt, Vader regretted becoming a cybernetic monstrosity. First, there's the constant pain and discomfort from his suit. Worse, however, is the psychological pain. Vader destroyed everything he had ever known for a chance to save Padme.
In the final days of the Clone Wars, Windu turned to Kolar again, calling on him, Saesee Tiin and Kit Fisto to arrest Supreme Chancellor Palpatine. Confronted by the Jedi, Palpatine threw off his guise and revealed himself as Darth Sidious. He ignited his crimson-bladed lightsaber and struck down Kolar.
Having covertly turned Kylo Ren—Anakin's grandson and Luke's nephew—to the dark side, Palpatine orders Ren to find and kill Rey, the last remaining Jedi and Palpatine's paternal granddaughter. Palpatine is ultimately defeated and destroyed by Rey.
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.
The dark lord revealed
Leaving Palpatine's office, Skywalker quickly went to the Jedi Temple and informed Jedi Master Mace Windu of Palpatine's true identity, though he was greatly conflicted about doing so.
Unfortunately for the galaxy, as Palpatine grew in power the shroud of the dark side fell over the galaxy, diminishing the Jedi's ability to sense the Force. Thus Yoda failed to realize how close to home the Sith really were - until it was far too late.