The first Jaws. If you'd ever been in deep water with hungries, the opening swimming scene should have made you nauseous. After that, the rest was a big action movie.

The first Alien. All around.

Nobody mentioned Saving Private Ryan. The first twenty minutes of that movie should have caused recruitment to dry up for a month. The best horror-antiwar movie I've ever seen.

Nobody mentioned the Killing Fields either. That only had about 10% of the impact of the real thing. That wasn't fictional horror though.