These are in no particular order:

F.W. Murnau

Perhaps no one was more ahead of their time than Murnau.  How amazing is it that his 1922 film Nosferatu is still popular and viewed frequently nearly 100 years later?   While Nosferatu was his most well known, he made 21 movies and many received great acclaim.  Included in his filmography is Faust, The Phantom, The Haunted Castle and 4 Devils.

Tobe Hooper

Credentials:  If he had directed only the Texas Chainsaw Massacre, that may have been enough to earn him a spot.  TCSM is largely regarded as one of the very few films that impacted and helped to shape an entire genre.  But, Hooper is also responsible for writing and producing a number of films in the TCSM family.  He directed Salem's Lot, The Toolbox Murders, Poltergeist and The Funhouse just to name a few.  He has directed 35 films and television spots, written a number of screen plays, produced 6 films and composed the music for several films, including TCSM.  

Alfred Hitchcock

As prolific as he is acclaimed, he directed 67 titles, produced 28 and wrote another 22. His work includes Psycho, Vertigo, The Birds, Torn Curtain and the list of classics goes on and on.  

Stanley Kubrick

Like Hooper, one film in particular may have been enough.  The Shining is as close to a modern horror masterpiece as we may ever see.  But, like Hooper as well, Kubrick directed a host of other classics.  His work includes The Shining, A Clockwork Orange, and Full Metal Jacket.