Probably Jaws, I still WILL NOT SWIM IN THE OCEAN, ANY OCEAN Since I saw this movie as a child. I actually get a little freaked out when I am out in deep water where I can't see the bottom in LAKES even. I start hearing that damm music in my head even though I know there is no way a shark can get me I still can't shake the fear that something is gonna eat me

Second place goes to the Exorcist, nightmares for weeks after seeing that one.