I am NOT a connoisseur of the genre*, but for a quick draft:
Night of the Living Dead
Dawn of the Dead
Invasion of the Body Snatchers (’78)
The Thing
Prince of Darkness
Evil Dead 2
28 Days Later
The Witch
Get Out
Mandy
Honorable mentions:
Night of the Hunter
The Beyond
What Lies Beneath
Pontypool
The Descent
The Vanishing
The Shining
Stuff like Cabin in the Woods, Scream, and Ghostbusters are excellent movies, but IMO really horror-themed comedies. Evil Dead 2 just got over the bar for me.
*Movies which this discussion is spurring me once again to finally get around to watching include: Audition, Halloween, Re-Animator, Kwaidan, Black Christmas, Don't Look Now, Black Sunday, and The Texas Chainsaw Massacre.