Most horror films, like most films in general, are rubbish, but I suppose that could be said of anything.Night of the Living Dead
are the two that stand out most to me, particularly the former; its ending is one of the most nihilistic attacks against modernity to be committed to film -- and has quite the aesthetic appeal, given the abrasiveness of the background noise accompanying the bleak still shots. The zombies are a metaphor for the mindlessness of the modern individual, their countenances reminiscent of figures in Munch paintings.
Unfortunately, the audio doesn't synch up with the video in the following link, but it's the only one I could find for the ending:http://www.youtube.com/watch?v=DXJrzCheiXwPsycho
has one of the more memorable scores. I've long searched for composers who approximate it in their works, and have found some satisfaction in Ligeti -- and, thanks to this forum, Alfred Schnittke -- but I often wonder if there's an entire compendium of 'horror'-style classical music from the 20th Century of which I am simply not cognizant.
As for M,
I don't see anything overtly 'horror' in its style, but German Expressionism as a whole was notorious for its 'horror' leanings. The Cabinet of Dr. Caligari
is an example, though I thought it relied too heavily on visuals to convey anything meaningful, and it strikes me as dated and silly.