For two of the best horror movies of all time, you need.to go back in time to the 1950s.
Night of the Hunter (1955): the original mad slasher movie, made long before the words mad slasher were ever invented, and the movie John Carpenter said he was hoping to do as good as when he made Halloween. Set during the depression, with Robert Mitchum (an actor who could make you cringe just by staring at you) as the killer, posing as a reverend with the words hate and love tattooed across the knuckles of his hands, chasing two young children across the state because he believes they know where $10,000 is hidden. The death scene when he kills his new bride, Shelley Winters, the mother of the two children, and the fisherman's discovery of her body by slowly coming to realize the seaweed he sees flowing at the bottom of the river is intermixed with the flowing long blond hair of the victim, are two of the best fright scenes you will ever see without seeing any blood.
The Bad Seed (1956): the first movie ever made where a child, a seemingly angelic and perfect eight-year-old played superbly by Patricia McCormick, is the sociopathic killer. Much of the movie is the interaction between the child and her mother while the mother goes through the stages of believing her daughter incapable of harm, to starting to suspect it might be true but denying it is, to finally realizing that her daughter is evil incarnate who kills those who displease her. The ending is an unexpected twist.
My third choice is more modern. For pure terror, you cannot beat Alien.