Scariest as a kid: The Child's Play movies. Man, I remember at department stores or the mall, they'd have a poster of Child's Play 2, and i would be legit scared to even sift through the posters and come across it. Took me quite a while before I could watch those. The Shining, especially the blow-job bear scene, was terrifying.
Scariest now: Event Horizon is up there. Anything cosmic or lovecraft-ian. Things that are beyond humanity and lurking. Another movie that does a good job with that "dread" feeling, more recently was The Dark and The Wicked. Instilled a real sense of dread, watched it only a few months ago, basically with my eyes covered most of the time in suspense. I also saw Paranormal Activity in theaters the day it came out, and even though we knew it had to be faked, it was still had that "did this happen?" suspense.. I couldn't sleep for WEEKS. I don't think any movie had a better roller coaster effect of suspense than PA, as it went from day to night. When the title showed "Day 3 Night" the theater was going bonkers in
Favorite is The Thing. Goes without saying. It's the GOAT of horror movies.