This list might change, but right now, here are the top 5 most voted for.
1. 28 Days Later (2002) http://5thin.gs/uN4RBk
With its visual and conceptual framework just dripping all of our fears of our world as it is today, 28 Days Later is most terrifying in its ability to take something far-fetched (zombies) and making them seem not just plausible, but likely and even inevitable.
2. The Exorcist (1973) http://5thin.gs/vpKtr1
With some many of us, this was a realitively subtle scary movie... not lurking monsters with blood splatters, but an invisible and inexplicable takeover of something sweet and innocent (a child). It is terrifying because it is shocking and about things that matter. Its fun to watch pretty, silly, foolish teens get picked off one by one. It is terrrifying to imagine your child turning into this.
3. Paranormal Activity (2007) http://5thin.gs/uwVXe1
The production quality of this sleeper hit, similar to Blair Witch, rely on its apparent artlessness... it's sense of being authetic. It's a powerful trick.
4. Alien (1979) http://5thin.gs/vjUheU
There is something truly terrifiying about stuff that is entirely unknown. And Alien represents that the way that any earth-bound monster couldn't.
5. The Omen (1976) http://5thin.gs/tLIC1R
The concept of evil is an interesting, and terrifying, one. The intentionality of harm, especially from a source so surprising, makes each of us feel the need to be viligant, for our death or destruction could come from ANYWHERE.