The most scariest enemy

[Krohm]

I'd say the water lurker. It's too easy to hide from the brute and the grunt, but the second you set foot in water the lurker is after you, even if it is slow and maybe easier to avoid than it should be.

Even if a grunt is starting to chase you and close, all you have to do is crouch behind something in the darkness, sometimes not even out of it's view and you can lose it.

Also, there's no waiting for the lurker to leave and IMO it makes the most intimidating sounds.