Yeah, Expelled, Hell House and Jesus Camp are pretty high on my list."Expelled" is probably the scariest movie I've ever seen, but mostly because I know how many people probably actually think it's true.
As far as actual scary movies, "The Ring" is the only one that ever actually scared me.
The American remake of the Ring or Ringu? I liked both, but slightly prefer Ringu if only 'cause I saw it first. And Sadako was far creepier in the original Japanese release.
I'm a huge horror fan and rarely find horror films frightening, but Session 9 is one of those rare films that was disturbing and got under my skin- if only they'd edited the last 10 minutes or so it would've been far more effective. Nakata's Dark Water was also an amazing horror film that was genuinely scary and melancholy (which made the horror all the more powerful). Alien terrified me as a kid, as did Carpenter's The Thing. The Haunting still has some scary scenes even after having seen it so many times- some of the best use of auditory cues to scare the audience.