Scariest Movie Ever?

What do you think is the scariest movie ever?  I'd love to know what it is and why.

For me, it probably is Arachnophobia.  I watched it when I was in 5th grade, and I lived in Florida at the time.  This means I saw gigantic spiders and spider webs that were the size of cars a couple times a week.  I was so terrified I couldn't sleep at night for a week, and I went two weeks without actually getting in the shower.

I'm trying to find it again, so I can convince myself that there is no way that what happened in the film can happen in real life.  Spiders still scare me..