I guess it depends what you define as monster. To me, the scariest would be the banshee. But no doubt most people wouldn't classify that as a "monster"
That's clown a little creepy. Idk what the most scary monster is, I don't watch many horror movies so idk what most of this is. Including the creepy clown. But when I was a kid, randal from mosters Inc was a little creepy lol. So did the bug from men in black. Uh but a real monster... Hm well I never liked vampires, always found them scary until they put them in girly love stories lol