I'm pretty sure that every single one of you there has a certain creature that either makes them either defecate in their undergarments or makes their pants stand like a wigwam. So, who are they? And why?
Just to establish something; this poll is general, which means that singling out certain individuals like, say, Dracula, just doesn't tell us enough. Go for the entire species. Also, nothing's stopping you from adding as much of the critters as you want.
Also, no beings of any kind created in the last 100 years. They are just too convenient and modern, and not scary enough.
My favourite monsters are the wendigo. They are almost like werewolves (which are second on my list) but much more human, which actually makes them more terrifying. Of course, like any good monster, they feed of of humans, but they are much more voracious than werewolves or vampires. Also, the way that they are created is much more unsettling and terrifying than for many other beings; feeding on human flesh for years, a human will become a wendigo. The best portray of wendigo I've ever seen was on the eponymous second episode of Supernatural.