It's obviously hard to judge since that fight was so poor. Obviously, Vader has a tremendous amount of physical strength compared to either Kylo or Obi, due to being a terminator from the neck down. His force powers at the time of A NEW HOPE were probably greater than Kenobi's, although it had not peaked as it had in Empire Strikes Back.
I'll give Kylo a possible raw Force advantage, but not with lightsaber. Mostly because, taking the fight with Rey into account, we have no idea as to whether or not he had even been in a lightsaber battle before. He's used it plenty of times as a weapon, sure, but I'm sceptical if he's clashed with someone else who was trained in similar arts. Vader, on the other had, has had countless saber battles.
I'm happy to concede that this could go either way due to perception. I trust Obi's tactical prowess and wit here.