We've had a number of threads about what techniques are good to spook, scare, and shiver your players, but here's another way to look at it. What creatures have you found particularly effective for chilling your players' blood? Not just as a matter of being too powerful for them, but in some way disturbing, unsettling, or just plain creepy. Personally, I love the leucrottas for their voice mimicry ability. Lots of options there.