Vader. Canon Vader has feats like easily crushing AT-AT walkers with the Force. And as for lightsabers... Kylo's only feats are almost dying to Rey twice, needing to be saved from one of Snoke's guards, and beating the Knights of Ren, who are featless.
Except the only reason he almost died the second time was because he was distracted. He beat Rey and was about to deliver a killing blow before he was distracted by Leia.
I think sabers clearly goes to Vader, as it's hard to measure just how good Kylo's skills are. Force powers are more debatable. I would favor Vader, because of the atst feat you mentioned, destroying an entire army single handedly in Vader Down, force waves powerful enough to shatter lylacs, which are impervious to blaster fire.. also, didn't Vader single-handedly kill a giant creature that wiped out an entire civilization?
xPRIMEx, why would beating Rey in a duel be impressive? Rey was getting her ass kicked by Luke in TLJ until she pulled a lightsaber on him. She has pretty much no training. You could say the same about ROTJ Luke, but Luke can be scaled from Vader. Rey and Kylo only have feats against each other.
On another note, in The Rise of Kylo Ren comic Kylo took out 3 Jedi just using the force alone. And I’m sure we’ll see the knights of ren get some nice feats in later issues as well.
Vader slaughters him
Vastly more skilled and powerful
__________________ "Happiness is a lie. Life is horror. The light is always dying all across the universe. The last star will flicker out someday, when it does, all that remains is shadow. And I will be its king!"'-Amahl Farouk