Two films stand out for me.
The first is Alien. The original one, which was imho a masterpiece of gradual suspense-building with one moment that frightened the bejasus out of me, in that it startled me and made me jump. That was the bit with the monster and John Hurt's stomach ...
But the one that had a lasting impact on me was Marathon Man. Lawrence Olivier as That Dentist. I've not been able to watch it since it came out. Just thinking about That Scene gives me the heebie jeebies. Seriously.
"Is it safe?" ... excuse me, I need to leave ... 