Psycho
An excellent example of the terror you can instill upon your audience through the use of pure suspense and an expertise with the film camera. Alfred Hitchcock's Psycho has the impossible capacity to frighten, unease, and shock audiences even 55 years after its release given that there are still some people that don't already know what is probably the most well known plot twist in film history.
This is easily the most important film for its genre that continues to show itself in the horror/thriller genre to this day.