Vader's innate talent with the Force would have made him more powerful than Palpatine, if not for Palpatine meddling in his life from a young age. As it stands, Palpatine's influence over Vader was too great for Vader's strength to overcome, forcing him to take Palpatine down when he was prone and unaware.
As Darth Vader, he was one of the strongest beings in the entire galaxy. But even then, he was never able to surpass his master, Darth Sidious. A Sith Apprentice can only become the master when they murder the one who trained them.
Darth Vader couldn't kill Palpatine before Return of the Jedi because he was everything to him, as that's how consumed with hatred Darth Vader was. It wasn't until he allowed himself to be filled with love for his son, and look past Palpatine at something more important, that Vader was able to finally kill his master.
As the Chosen One in his prime, it's quite possible that Anakin would have had a strong chance of defeating Palpatine aka Dark Sidious. Palpatine himself told Yoda before their Revenge of the Sith duel on Coruscant that Darth Vader would become more powerful than either of them as he continued to grow in the dark side.
Admittedly, Rey skills couldn't possibly be as developed as Anakin's or Luke's. Nevertheless, considering how fast of a learner she is, and how unique the powers she possesses are, it's not a stretch to say that Rey could be a more powerful Jedi than Anakin and Luke Skywalker.
When it comes down to both combatants in their prime, Yoda was superior to Darth Vader. That debate extends even further to prime Anakin Skywalker as well. He would have been outmatched by Yoda for many of the same reasons Obi-Wan outmatched him, and more.
Obi-Wan faced off against Darth Vader four times and physically beat him twice while beating him through his Force development in a third fight. Obi-Wan Kenobi had the high ground that led to Vader's disfigurement and becoming the ultimate Star Wars cyborg.
For his disobedience, the Emperor punished Vader and resolved to teach him a lesson in the dark side of the Force, removing his cybernetic limbs and having him dumped on Mustafar by a squad of death troopers to either strengthen his Sith apprentice with pain or leave him for dead.
Yep. Darth Maul didn't really say or show straight up that he "feared" Darth Vader, but he openly stated his belief that he could not defeat Darth Vader without the help of the Jedi, therefore on his own.
Vader had been using all his energy to intimidate Luke, so when the actual fight began in a flash, he was already weak and panting. This leads to another huge factor, which is the age difference. While Luke was a young and spry 23-year-old, Vader was reaching his late 40s.
However, once Luke fully embraced his power in Star Wars: Return of the Jedi, he proved that he was indeed stronger than Darth Vader and struck his father down.
History. A dark side warrior with a mysterious past, Kylo Ren was neither Jedi nor Sith, but a product of both sides' teachings. Once an apprentice of Luke Skywalker's, he killed his fellow students and drove Skywalker into exile, becoming a First Order warlord and servant of Supreme Leader Snoke.
The only Sith Lord who actually killed Vader, Darth Sidious kept his apprentice in line for over two decades. His true advantage was Force lightning, a power which would wreak havoc on the suit keeping him alive.
Legend had it that Yoda—a Jedi who became Grand Master—was trained by N'Kata Del Gormo. A Force-sensitive Hysalrian, N'Kata Del Gormo was trained in the ways of the Force and achieved the rank of Master within the Jedi Order.
Kylo lost to Rey in TFA because he was heavily wounded and was holding back against her due to Snoke's request that he bring her back to him alive. He is also stated by Palpatine in the Secrets of the Sith book to be “equally as powerful as a real Sith despite not being one”.
As further highlighted by Vader and Kylo's redemption stories, it was a lesson that Kylo Ren never fully learned himself, which makes Darth Vader the more powerful user of the Force between the two of them in Star Wars.
This caused him to attempt to assassinate him in his sleep. Needless to say, Kylo Ren is almost definitely a significant threat. However, we cannot definitively say that he is more powerful than Luke. Kylo's powers are vast but they are still highly unbalanced and difficult to control.
As the grandson of Anakin Skywalker (on his mother's side), one of the most powerful Force users in the galaxy, Kylo Ren has the appropriate bloodline to destroy Palpatine. It was a redeemed Anakin Skywalker who killed Palpatine the first time, and the same midi-chlorians flow through Ren's veins.
That said, regardless of Palpatine's true intentions during that fight, Mace Windu is powerful enough to defeat Palpatine. Windu combines incredible lightsaber skills with extensive knowledge of the Force, not to mention unique abilities such as the now-Star Wars canon Shatterpoint.
Multiple times throughout the canon Marvel comics series titled Darth Vader, Palpatine attacks Vader every time he shows weakness. He often refers to Vader as a tool that can be discarded when he is no longer of any use, and yet still Vader obeys.
