Darth Vader may be more powerful, but Kylo Ren was a more dangerous Star Wars villain. As the son of Han Solo and Leia Organa, Ben Solo was meant to follow the footsteps of Luke Skywalker and be a Jedi. But like his grandfather, Anakin Skywalker, he fell to the dark side and become evil.
As a villain, Kylo Ren was obsessed with Darth Vader; he tailored his whole look to match the iconic Sith Lord, donning a mask and a cape just for aesthetic. Conveniently, he disregarded the fact that at the end of Return of the Jedi, the Sith Lord redeemed himself, which ironically also how his arc ended in Emperor Palpatine's plan to amass more power and ultimately be the ruler of the galaxy.
In of pure power, Darth Vader was more powerful than Kylo Ren. Aside from being one of the strongest with the Force, it's also worth noting that Anakin did get more training from both the Jedi and the Sith making him better skilled at fighting. Despite this, Kylo Ren was a more dangerous villain compared to his grandfather because of his instability. As seen throughout the Star Wars sequel trilogy, Han and Leia's son was prone to emotional outbursts making him a threat to pretty much anyone he encountered. In one of his most memorable meltdowns in BB8 in Jakku. Upon further hearing more information about the escape, he took his frustrations out on his subordinates despite him being merely the bearer of bad news.
Since Kylo Ren was extremely volatile, he acted in the moment; his emotions dictated his decisions especially during intense situations. At times, this worked for the better in his decision to betray Supreme Leader Snoke after getting berated and humiliated by him countless times throughout their time together during the sequels. But for the most part, his inability to keep himself in check posed a threat even to his allies as they could easily be victimized by his rage-fueled actions. He was so unpredictable, one could never know what he'll do next. At least Darth Vader only really went after his people if they do or say something outright insulting to him as shown in the original Star Wars movie.
In hindsight, Kylo Ren had all the makings of a great villain. The way he turned to the dark side is frankly more compelling than Anakin's in the prequels and his bloodline already made it easier to be invested in his character. Unfortunately, the Star Wars sequels failed to execute an otherwise great journey through the films — albeit it's worth noting that Kylo Ren already had the best overall arc in the Disney-produced installments.
