That's a tough question. I've always believed that a horror film had to have an element of the supernatural to be classified as such. But the line is blurry, and today films like "Hostel" (2006, Eli Roth) and "Saw" (2004, Gary Elwes) just revel in sadism and gore. They are in, what is to me, a grey area. As are films such as Invasion of the Body Snatchers (1978, Donald Sutherland), which is technically science fiction, and Silence of the Lambs (1990, Anthony Hopkins), which is technically a psychological thriller. This last type of story can be confusing, as it's often unclear as to whether we are witnessing supernatural things or character hallucinations.
Of the classical/supernatural horror films, Francis Ford Coppola's "Bram Stoker's Dracula" (1992) is perhaps my favorite
That's probably because I like the story so much, having read the book 3 or 4 times. And because Coppola embellished the story with scenes that were not in the book, yet logical; serving to fill-in what Stoker could not or would not or did not want to say. My only complaint about this film is Anthony Hopkins' portrayal of Van Helsing. I just don't get it. And I don't like it, much as I like Hopkins as an actor.
Other favorites in this genre, not an any particular order, are Vampyr (1932, Carl Dreyer), Nosferatu (1922, Max Schreck), Carnival of Souls (1962, Herk Harvey), Pumpkinhead (1988, Lance Henrik), Angel Heart (1987, Mickey Rourke), The Fog (1979, Jamie Lee Curtis), The Shining (1980, Jack Nicholson) and The Exorcist (1973, Linda Blair).
Of the psychological thrilles, Alfred Hitchcock's "Psycho" (1960, Anthony Perkins) is probably my favorite. The Bates Motel still stands on the Universal Studios lot.
Other favorites in this area are Sisters (1973, Margot Kidder), Jacob's Ladder (1990, Tim Robbins), The Cabinet of Dr. Caligari (1921, Conrad Veidt), The Machinist (2004, Christian Bale), The Texas Chainsaw Massacre (1974, Tobe Hooper), Misery (1990, Kathy Bates), The Name of the Rose (1986, Sean Connery) and No Country For Old Men (2007, Tommy Lee Jones).
And of the science fiction horror films, my favorite is Alien (1979, Sigourney Weaver). This is the stuff of nightmares.
Other favorites of this type are Frankenstein (1931, Boris Karloff), The Thing (1982, Kurt Russell), The Devils (1971, Oliver Reed), The Fly (1986, Jeff Goldblum), and Aliens (1986, Sigourney Weaver).