Scarriest thing, really?!!! Scarrier than facing Darth Vader, the man who murdered your father and your mentor?
Apparently so according to Luke himself:
“I’ve seen this raw strength only once before. It didn’t scare me enough then. It does now.”
In that dialog he is talking about Ben, so I guess he sees the potential in Ben to be even worse that Darth Vader or the Emperor
Vader was a quadruple amputee in his 50’s and had weakened himself to unimaginable extent after he lost to Obi-Wan in Mustafar. So Kylo Ren in terms of raw strength might’ve been in Vader’s league. However, I don’t think Luke had that much time and was enough in tune with the force both times he faced Vader to be able to sense his raw strength. I think he was more focused in Vader’s feelings in RotJ and in ESB he was just shitting his pants, really.