There are easily a good 100 or so horror films from the 70's that can overthrow most modern horror in terms of suspense, gore, social commentary, innovativeness and most of all atmosphere.
There are the obvious 70's horror films like John Carpenter's Halloween, The Texas Chainsaw Massacre and The Exorcist then there are the 2nd tier of 70's horror like Black Christmas, George Romero's Martin, and Suspiria.
Hollywood has laid waste to most of these good names with remakes, rehashes and re-hacks that most common horror viewers are unaware that The Hills Have Eyes, Last House on the Left or The Crazies are remakes.
My unconventional top 70's horror is as follows (By unconventional, I mean you might not have heard of it)
Blood Spattered Bride (72)- Carmilla Karnstein story, A newlywed wife undergoes twisted sexual abuse from her perverse husband and feels trapped. She stumbles (in the most odd introduction of characters) upon Carmilla, a forgetful and lost, beautiful young woman who brings dominating sexual and bloody havoc to the household. Strong surreal moments and excessive blood letting, unpredictable ending very impressive for the early 70's. Highly recommended 10 out of 10. A perfect horror film in every sense.
Shockwaves (77)- The quintessential Nazi Zombie film, starring Peter Cushing. A boat is shipwrecked on a seemingly deserted island, that is the old experimenting grounds of SS killing machines. They awake from their slumber and kill again. Fantastic cinema photography, the suspenseful build up to the killings is treated as fair and intelligent to the viewer, equivalent to Carpenter's delivery in Halloween. The ending is very disturbing and leaves a grim feeling, superior film that deserves more attention. 8 out 10.
Rabid (77)- Another David Cronenberg gore-fest. Adult star Marilyn Chambers under goes radical plastic surgery after a motorcycle accident then its all "body-horror" meets Night of the Living Dead. Odd deaths that involve a tentacle from the armpit. Comparable to Romero's The Crazies with trigger-happy Military vs.Quarantined Citizen. Chambers proves her acting abilities outside the adult world as the terrorized and unknowing murderess. 9 out of 10.
Let's Scare Jessica to Death (71)- Early 70's ghost story, is Jessica loosing her mind? Beautiful cinematography, thick atmosphere that you can cut with a knife and a haunting minimalistic score.. A post mental Jessica and a few friends getaway to the country in a creepy house with an orchard that needs upkeep. They find a beautiful seance-loving hippy squatting in their new house and the towns people are scabby octogenarians that pose an immediate threat. Gorgeous film, atmosphere with a capital "A". 10 out of 10.
Tourist Trap (79) - Early David Schmoeller (Puppet Master) peculiar night-time slasher,sick drive-in horror finally on DVD. Originally and inappropriately released as a PG film. Stranded teens encounter help from a local man (Chuck Conners) and take lodge at his creepy wax museum and have a run-in with his demented brother, a maniacal telekinetic mannequin hording-villain, unlike anything ever scene in horror. Weird and unique death scenes, bizarre ending that stuck with me for years. 9 out of 10.
Silent Night, Night Bloody (74)- Crazy people used to reside in an old house, have they resided there all these years? Watch for the beautiful sepia toned asylum flashback. The House on Haunted Hill remake lifted this scene. 7 out of 10.
Isle of the Damned (76) aka Who Can Kill a Child. Hitchcokian elements inspire this evil child horror film. Low on gore, heavy on suspense. The first documentary style segment is unnecessary however the film build and builds as the town closes in on the unsuspecting couple. 8 out of 10.
Baba Yaga (73)- AKA The Devil Witch. A young female photographer cross paths with Baba Yaga, the mythos of an evil witch, at 3AM on a foggy night in a grimy cab (not spooky at all) . After wards the photographer's camera becomes cursed and is an object of murder. Poor dubbing, great imagery, cool concept of photographs within the story telling. Very psychedelic. 8 out of 10.
Daughters of Darkness (71) Harold Kumel directs this big budget, Vampire, Euro-Horror with a classy finesse, more dialogue driven and focusing on the drama and turmoil of another married couple with a Countess that comes in between their marriage. 9 out of 10.
Vampyres (75) Part Vampiress Gothic-Horror and part lesbian-erotica. Two beautiful young females lure men and women back to their castle to commit seduction and murder. Gained a cult following. Great restored copy by Blue Underground. Not for the Squeamish. 9 out of 10.
Private Parts (72) One of the most strange love scenes in any film ever, (spoiler, man + blow-up doll) but don't let that scare you. Paul Bartel's Private Parts is a creepy hotel horror with even creepier tenants, more of a bizarre character study than actual scares. Great restoration and much better than Howard Stern's film Private Parts. 8 out of 10.