Blair Witch was bad, but Paranormal Activity takes the prize for total worthless idiocy on film. When the girl is taken into the hall from her bed, and the guy runs after, and we are left watching an empty room while she screams for ten minutes....I was cheering for the ghost to just kill them already!
I tell the truth that PA TERRIFIED me for months. (And pretty much scared me away from the entire horror genre indefinitely.)
Then again, I never thought any other "horror" film to be all that scary. Halloween was good, but it didn't really scare me, and neither did any other slasher film. The Exorcist also didn't really scare me.
The problem with those movies was the fact that you could see everything.
The fact that Blair Witch and PA didn't show anything is what made them so scary to me. (Helped that I saw PA in the theaters with a bunch of other people who were really into it; I saw Blair Witch at home during the day, so while it was somewhat disturbing, it didn't scare me as much.) The unknown is the scariest thing of all for me, and the makers of PA really knew how to do that well.
Besides, you want to talk about worthless idiocy... I assume you've never had to sit through Glen or Glenda?