Arachnophobia. I saw it when I was 7, and for the next 10 years I was convinced that anything with 8 legs was instant death. That movie made me so paranoid that I would check under the toilet seat every time I used it. I was convinced they would get me in the shower, or crawl into my shoes, or hats, or food; if I saw a spider anywhere, I would run screaming from the room. And the insomnia from spider-based nightmares? Forget about it. I'm fine with spiders now, but I'm pretty sure that my parents wanted to kill the film-makers for just how much that traumatized me during my formative years. Apparently it's a comedy, but it will always be the most terrifying movie I ever saw.
Jaws was pretty bad too. I would be scared of running into sharks in the deep end of swimming pools. Or my bathtub. It was ridiculous. If I was taking a shower and closed my eyes and got an image of a shark, I'd have to leave the shower immediately. Actually, between being afraid of sharks and spiders getting me while I bathed, I spent a good portion of my childhood extremely filthy. Not much has changed, really...