Top Ten

I Walked With A Zombie
Vampyr
Blood And Black Lace
Cat People
Black Sunday
Night Of The Demon
The Pit And The Pendulum
Les Diaboliques
The Innocents
Plague Of The Zombies

The Innocents
Invasion of the Body Snatchers-1955 version, the remakes were crap.
The Uninvited-1940's version with Ray Milland
Original The Haunting
The Abominable Snowman-Peter Cushing version
Bride of Frankenstein
Psycho
Homicidal
Scared to Death-An old campy turkey with Bela Lugosi. I saw it when I was 6 or 7, and it scared the living crap out of me.

Horror Of Dracula
Bride Of Frankenstein
Godzilla, King Of The Monsters
Creature From The Black Lagoon
Psycho

in no particular order:

The Cabinet of Dr Caligari
Frankenstein
The Pit and the Pendulum
Night of the Demon
Black Sunday
Psycho
Vampyr
The Horror of Dracula
The Haunting
Kwaidan

This isn't really a top 10, I just tried to pick representative examples from some of the classic sub-genres of horror, and even restricted to 10 I can not fit them all in. Even so, I feel like I skipped the drive-in/matinee movies of the 50s, and for that I would have picked either Them or one of the Ray Harryhausen movies, and I also skipped the Japanese giant monster movie, for which I would have picked Godzilla obviously.

VERY tough to just list five, but with all these suggestions, you'll find plenty to choose from. My Top 5:
NIGHT OF THE LIVING DEAD (1968)
NIGHT OF THE DEMON (1958)
BRIDE OF FRANKENSTEIN (1935)
THE INCREDIBLE SHRINKING MAN (1957)
THE BODY SNATCHER (1945)