Kylo Ren because of the idea behind the character. A young, confused, and powerful kid, who has been manipulated into the path of the dark side. He wishes to be just like Darth Vader, but is no where near as skilled and powerful as him and everyone knows it, even if he tries to make himself appear menacing with his mask and voice changer. That’s probably why they hired Adam Driver, because he didn’t look very intimidating and I feel that that was something done on purpose to convey the difference between him and Vader. Definitely the best handled character in the sequel trilogy