I would have been watching a selection of the old Universal flicks from the '30s - Dracula, Frankenstein, The Mummy etc - on TV from a fairly early age, growing up in the '70. I really don't know when or exactly what would have been my first though. These would have been joined by some of the Hammer and Amicus productions a little later.
I do recall staying up alone - at an age when staying up alone was just about allowed, but still a novelty for me - to watch what was perhaps the UK TV premier of The Omen (1976) on a crackly black and white set in a dark room and being blown away by it. I became obsessed with the whole biblical Revelations mythos side of it for quite some time after. It certainly had me reading more of the bible than endless Sunday school ever had.