The original 'the Haunting' from the '50s or '60s in black and white.

No special effects by modern standards but a terrifying film nonetheless. The first time I saw it with a bunch of friends at uni and we had to switch it off halfway because some of us (not me I hasten to add ) couldn't take it anymore.

I saw Jaws when I was about 6 or 7 I think, gave me nightmares for ages. The simple ones are still the best.