I'll give you some of the highlights from each decade. From what I've seen at least.
L'Inferno (1910's)
Dr. Caligari, Nosferatu, The Phantom Of The Opera (1920's)
Dracula, Freaks, The Mummy, King Kong (1930's)
The Mummy's Hand, The Wolf Man, Cat People (1940's)
The Thing From Another World, Invasion Of The Body Snatchers, The Fly, A Bucket Of Blood (1950's)
Eyes Without A Face, The Little Shop Of Horrors, Carnival Of Souls, The Haunting, The Birds, The Last Man On Earth, Night Of The Living Dead (1960's)
The Bird With The Crystal Plumage, The Exorcist, Shanks, Black Christmas, The Stepford Wives, Deep Red, The Omen, Carrie, The Sun That Dreaded Sundown, Eraserhead, Tourist Trap, The Amityville Horror (1970's)
The Fog, Prom Night, Altered States, Creepshow, Videodrome, Cujo, The Dead Zone, Phenomena, The Fly, Henry: Portrait Of A Serial Killer, Monkey Shines, Shocker, Begotten (1980's)
Jacob's Ladder, Twin Peaks: Fire Walk With Me, Braindead, Candyman, Interview With A Vampire, The Stendhal Syndrome, Pi, Ringu, The Night Caller, Ring 2, eXistenZ (1990's)
Ring 0: Birthday, Final Destination, The Others, Dark Water, The Eye, The Ring, One Point O, The Orphanage, 1408, Lake Mungo, Mirrors (2000's)
Dear Mr. Gacy, Julia's Eyes, The Skin I Live In, Shopping Tour, The Babadook, The Voices, Thelma, Hereditary, Dachra, Doctor Sleep (2010's)
No 2020's yet.
I decided to mix mainstream and more indie/obscure, but excluded the most obvious ones like Psycho, Alien, The Shining and Saw. Even The Conjuring.
I came close to recommending Day Of The Woman, Cannibal Holocaust and A Serbian Film, but those depend on whether you and your girlfriend have very strong stomachs or not.