One can argue who is more influential in horror movie and there are people who will disagree with this list, many people were left off this list, and maybe some changes could have been made. Its not fool proof and is the opinion of one man.
This is one view on the most influential directors in horror history.
Steven Spielberg is a household name, for many reasons. He has made huge accomplishments as both director and producer and in 1975 he added a horror movie to that list. Jaws is one of those movies that at the time became a surprise hit. Many films of that same nature, monster movies, had come and gone to little fanfare and horror in general still wasn't huge to the general public. Then Spielberg strolls on by and drops Jaws into the laps of the viewing public and horror becomes mainstream. Spielberg has gone on to produce horror hits, like Poltergeist, and to direct films on the cusp of horror such as Jurassic Park. But back in 1975, he helped shape the landscape of horror by making it far more mainstream than it had been in years.
When horror fans think of Tod Browning they turn to two titles, Freaks and Dracula. By today's standards, Tod Browning's work is tame and is mostly held in high regard only by fans of the era and horror film aficionados. The reason why he is on the list is due to his ability to push boundaries and the limits of the audiences he made films for. Take, for example, Freaks. The film was poorly received, considered too extreme even with extensive cutting of the footage, even being banned in the UK for 30 years. It wasn't until the 60s that the film took on cult status and just dues were served.
Friedkin is on the list for mainly one film, 1973's The Exorcist. The film was met with mixed reviews and over time grew more and more popular. The film was considered very extreme graphically, some theatres going as far as providing barf bags. The film was steeped in superstitions and said to be cursed as several members of the cast were injured on set, as well as one member of the cast, Jack MacGowran died of influenza shortly after filming.
There is a small list of films directed by John Carpenter that make him a big deal amongst horror movie fans, like Halloween, The Thing, The Fog, and They Live. All of his films, both horror and not, have followings and many directors since him will cite him as a major influence. With Halloween, he helped perfect the art of the slasher film and paved the way for directors in the 80s. Carpenter created the basic shape of horror for the 80s.
If John Carpenter shaped the mold of the 80s, Sam Raimi filled it in. And blood was his filler of choice. The 80s was ripe with horror, and many of those films created icons from the film. One such icon was created by Sam Raimi in the trio of films starring Bruce Campbell as Ash, The Evil Dead, Evil Dead 2, and Army of Darkness. With these films Raimi solidified himself as a gore god, pushing the envelope as to how much blood could be used in a film.
Italy, when it comes to movies is very well known for its time making “spaghetti westerns.” Italian horror of the 60's and 70's, on the other hand, isn't well known to mainstream audiences, but it has influenced horror movies greatly. One such director of Italian horror and Giallo fims is Dario Argento. Giallo, meaning yellow, is a term used for psychological thrillers/horror, named such for the yellow covers of the books in that genre. Argento is one of the biggest names in the game, with films like Susperia and Tenebre.
The first film that comes to mind when Tobe Hoopers name comes up is Texas Chainsaw Massacre. That film, by far, is one of the most influential films in horror history spawning countless imitations and influencing many directors to come after. Another film by Hooper to create an influence, and reach a wider audience, was Poltergeist. Poltergeist helped shape the landscape of haunting movies.
Its so easy just to type King Kong and leave it that.
There's not much to say on the subject, the film King Kong is one of the most, if not the most, influential films for horror and science fiction, which it can be classed as both. Like with the previous statement about Tod Browning, one can look at this film and think it tame, boring even, but in it heyday it was a brilliant piece of filmmaking, cited by many in the special effects industry as being a hug influence. King Kong is a huge deal.
One of the most prolific people in the horror industry today, hell of all time is the king of B-Movie horror Roger Corman. According to IMDB the man has directed 57 titles and produced 406. Roger Corman has single handedly fueled the independent horror scene and helped keep b-movies alive. Whether a person likes his movies or not, whether they consider them good or not, they are entertaining. Corman shows the world how fun horror can be, especially in a film world, which can take itself a little too seriously.
In a similar vein as King Kong not much needs to be said about Hitchcock. He was the king of pushing boundaries, Psycho and Birds are prime examples of that. Plus he influenced, and was influenced by the Giallo films during that time period. Its quite certain that the horror movie industry would look completely different without Hitchcock.