Darth Vader wins. Kylo Ren wouldn't last long enough to finish requesting an autograph. Rogue One finally showed what Vader can do, and the only way Kylo could stand a chance against that would be if he magically turned Vader into a computer console.
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.
1 Luke Skywalker
Having faced his father once before, Luke was familiar with his tactics and used the force to gain an upper-hand. In Star Wars Canon, only Luke Skywalker can be acknowledged as the one who truly defeated Darth Vader.
Out of all of the Jedi Masters in the storyline of Star Wars, the only character that defeated Vader decisively multiple times was Obi-Wan Kenobi. He was never the strongest Jedi regarding his combat prowess and Force powers, but he had the patience, discipline, and wisdom of a true Jedi Master.
It is debatable whether Kylo Ren ever earned the title of Sith Lord, but he does show some of the most powerful uses of the dark side of the Force in the Star Wars films.
Darth Nihilus is an iconic Sith Lord that many Star Wars fans adore. He is most known for the Sith Lord with one of the best “looks” since he wears a skeletal mask with a dark shroud.
Some time later, he faced the Sith Lord Darth Vader. Infil'a put up a fierce fight against Vader, almost killing him by throwing the already damaged Sith Lord off of a cliff. Despite severe damage to his suit, Vader survived the fall, and repaired himself with parts from Infil'a's droid, Arex.
Yoda will most likely win in a fight against Darth Vader because he is more skilled with a lightsaber and has the experience and feats that allow him to trim whatever size and strength advantage Darth Vader has over him. In fact, Yoda nearly defeated Palpatine, something that Vader could never do.
Luke Skywalker
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.
Vader, the only power who overrules him is papa Palpatine.
Darth Vader's Insecurities are His Biggest Weaknesses
In the end, virtually all who have made the attempt to use Vader's cybernetic nature against him have ended up dead or defeated. The elite few who have had the upper hand, even before his conversion to mechanical monstrosity, have done so by dominating him mentally.
You could throw every Sith Lord and beings with the force his way and they'd all lose. One by one or all at once, the Hulk would beat them all. You can take your pick with which Hulk Darth Vader fights. Gray, regular, World War Hulk, or any other.
Even though Kylo Ren uses a red lightsaber, harnesses the dark side of the Force, and wears a cape, he is not a Sith. Because of that, he can never hold the title of Darth. Instead, he was just a chaotic mess of the dark side (until he eventually found his way back to the light).
For many, it seemed like a copout as we were all trying to guess who Rey was, who her parents are, and where all of this power came from. The Rise of Skywalker novelization has confirmed exactly how Rey was able to take down Kylo Ren with ease and it all has to do with Chewie.
Being the grandson of Darth Vader and nephew of Luke Skywalker, who we know to be powerful Force users, likely gave him a boost since the Force is strong in their family. Good or bad, Ren has received training Rey hasn't.
This ensured that the Sith could hone their skills in secret without fear of betrayal from rival Sith factions. More cunning and devious than all but the most attuned Force-sensitive beings, Darth Bane could more than likely defeat Yoda in a battle of both strength and wit.
While Darth Vader can make the fight interesting with the use of the Force, Thanos is just too strong and durable for the Sith Lord to defeat.
It's implied that for the rest of his life, Lieutenant Crest never fails Vader again, so it really was "for the last time."
The Force: As the Chosen One, Vader was immensely powerful and had an amazingly strong connection to the Force, stronger than any Force-user ever in existence. He also possessed the potential to according to Obi-Wan Kenboi become the most powerful light and dark side Force-Sensitive being in the history of the galaxy.
The lightsaber duel didn't last long due to Luke's lack of training with the Jedi weapon and the Force, so Vader effortlessly snatched his old lightsaber from his son. Luke managed to escape before Vader could kill him, but Vader was quickly the winner of the fight itself.
Truly, the most powerful Sith Lord was Darth Sidious, better known in his public persona of Chancellor (later Emperor) Palpatine. Through cunning and manipulation, Sidious killed his master to claim the mantle of Dark Lord of the Sith.
1. Emperor Palpatine. Darth Sidious infiltrated the heart of the Republic and twisted it into the Empire, sparked a terrible war that engulfed the entire galaxy in fire and ash, corrupted once-noble Jedi to his cause, and executed Order 66 to erase the Jedi Order from existence.