Def not the whomping willow, I don't care how dangerous it is, I'm not afraid of a tree lol. I think the Horntail is dangerous but pretty cool, not scary. The dementor is scary, but just make sure you cast a patronus and you're fine. It's between acromantula and basilisk. I'd have to say acromantula because I have a fear of spiders and even if you try and run away hundreds of smaller, faster spiders will try to catch you just like in CoS.