Wes Craven died last night, succumbing to brain cancer at the age of 76. I'm hard pressed to think of a more important American* horror filmmaker in his lifetime.
Between The Last House on the Left (1972), A Nightmare on Elm Street (1984), and Scream (1996), Craven made genre-altering flicks across three decades. The Last House on the Left's gritty aesthetic and hyper-horrifying subject matter (the rape and torture of a teen girl followed by the soul-deadening revenge her parents take) helped spawn a whole subegenre of horror films. Aside from Halloween, A Nightmare on Elm Street is the only of the slasher flicks from the late-1970s, early-1980s worth re-watching. And Scream's post-modern deconstruction of the genre is almost inarguably the most influential horror film of the last 20 years; its impact is felt every time someone talks about "the rules" of such films.
And this is to say nothing of impressive efforts like The Hills Have Eyes (1977) and Red Eye (2005). Other directors may have stronger individual efforts in the genre (John Carpernter,** in particular) and some may have made a deeper impact on specific subsets of horror (George Romero is, was, and always will be the king of the zombie film). But for my money, Craven's horror resume as a whole over the last forty-plus years can't really be topped.
If I'm going to recommend just one Craven film other than those listed above, it'd probably be Wes Craven's New Nightmare (1994). Along with Last Action Hero (1993), this meta-horror film—in which Freddy Krueger intrudes into "our" world by tormenting the actress who "killed" him in the original film—helped set the stage for Scream and other self-aware genre work that has proliferated over the last couple of decades.
*I specify American because I don't feel like dealing with those who prefer to read their movies and will respond "Actually, what about Argento or Bava?" My knowledge of giallo is incomplete. For what it's worth, I think Craven's better than them anyway.
**John Carpenter is probably the better director in toto. However, much of his best work isn't within the genre of horror: They Live and the Escape films and Big Trouble in Little China aren't horror flicks. Halloween and The Thing are pretty boss, though.