Self-explanatory. Anyway.

To me, it's a tie between the Butcher and the Pretty Lady ones. He looks vicious in his butcher mask, it's clearly the most rotten one, his eyes look most murderous, and his messed up teeth adds to the ugliness of that mask.

The PL one, well, it's a little hard to explain why I find it as scary as the Butcher mask, in it's own way. Maybe it's the lips. Maybe it's the feminine eyebrows and eyes. Whatever it is, I feel it's the scariest-looking one in the intros where he "replies" by yelling "Aaarrgh!". It is reminiscent of the Joker's face, too.

As for the Killer mask, I like it. It's got that creepy Joker-like smile, and not being able to see his eyes well adds to the creepiness, but I don't find the mask as impactful as the other two. I think it's because it looks more...fake, compared to the other two.