Don't know if it qualifies as a "Halloween" movie, but there is only ONE movie that really scares the shit out of me, no matter how many times I see it, is...
The Shining
From the intro scene with the car driving in the mountains and that eerie music track, I get chills to the very end. Great flick!
(not to be confused with this Shining...
http://www.youtube.com/watch?v=KmkVWuP_sO0)