The scariest? That has to be subjective...
The first movie that really freaked me out... I sneaked out of bed and went downstairs late at night when I was about 7 years old, sat alone in the dark in front of the TV watching the original "The Blob". For nearly the next half dozen years I would always look under behind doors, under the bed, and anywhere it could be hiding.
About the same time I snuck downstairs and watched the original War of the Worlds... and that scared me.
The first time I watched the Exorcist when I was about 10 freaked me out too.
As for recent movies... The Grudge was one hell of a creepy film, it wasn't really too scary, but it was one of the most creepy films I've ever seen. The Descent was a great flick as well.
Alien, The Omen, Phantasm, Amityville Horror are all classics and were scary when they came out....