For me:
The Ring
The Descent
Alien
The Texas Chainsaw Massacre (original)
The Others
Saw
Arachnophobia
Seven
The Ring
The Descent
Alien
The Texas Chainsaw Massacre (original)
The Others
Saw
Arachnophobia
Seven
The scariest movie I have ever seen with out a doubt would be the original version of "Black Christmas", starring Olivia Hussey, John Saxon and Margo Kidder, It's an older movie dating back to maybe 1980 or so. They did it a real disservice by remaking it a year or two ago.
It is so freakin' suspenseful, you are squirming in your seat 1/4 of the way thru the film.
At one point, I screamed like a little girl and threw my soda into the air. It isn't even that bloody a film, very little blood actually, but it creeps into your mind and cold tendrils wrap around your heart and lungs.
Nothing I have seen before or after can compare to that movie. I own it on dvd and have never been able to watch it after that first time. I've lent the movie to several friends to watch, and all of them returned it telling me they were scared shitless watching it.
It has consistently been on the top ten horror list since it came out.
The original, "Texas Chainsaw Massacre" was also deeply disturbing for me.
"Joyride" with Paul Walker and Steve Zahn is way up there on my list of movies that really shook me.
Aliens needs a mention on that list, too.