Banshees due to their scream.

Banshees are lovely XD

I remember doing the Multi-player with my friends and hearing a collective groan when we heard a Banshee scream.

The Banshees are the most terrifying looking creatures that I've seen in the trilogy. They are like something out of a nightmare. If I would see one, I would scream and run away as fast as I could. I don't think I've ever seen a creature more terrifying!!