It's tough --

THE HAUNTING 1963
THE INNOCENTS 1961
AN AMERICAN WEREWOLF IN LONDON (best werewolf movie ever)
THE WICKER MAN (original, if it qualifies as horror)
SANTA SANGRE (surrealist director Jodorowsky's arthouse homage to slashers and director Tod Browning)

My two favorite horror anthologies
TALES FROM THE CRYPT (Amicus 1972)
CREEPSHOW

My two favorite silent horrors
THE CABINET OF DR CALIGARI
THE UNKNOWN

I agree about HENRY PORTRAIT OF A SERIAL KILLER, one of the most disturbing serial killer movies ever made. Very real.