I am really surprised how few people said Chrysalis. I chose discord personally because he really has no interest in killing you, just making you miserable. Though Chrysalis would definitely be second since she is really freaky looking, and would completely crush your spirit before killing you. I mean seriously, imagine if she came up to you disguised as your best friend and tried to kill you, and what if you never even knew it wasn't your friend?! On my opinion, that would be the scariest thing of all of them. Tirek would be third, since he can kill you instantly. Then Sombra, since he would be the scariest to fight. Then Nightmare Moon, since she is so powerful. Though Starlight would be last, since she would be the easiest to take down, NOT EASY, easiest, and she would probably be the only one who might let you live without torturing you.
*Edit:
Also, Chrysalis can MIND CONTROL YOUR ACTUAL FRIENDS!!!!!!!!!!!
@PiratePony Did you forget that Chrysalis can MIND CONTROL?!!!!!!!!