I agree about Alien, it is one of the best. Two you seldom hear about but which rank among my favourites are:
Nomads
Rosemary's Baby
Nomads is not well known, possibly because no one thinks a gothic or horror film with Pierce Brosnan is likely to be very good. In the opening scene Pierce Brosnan's French anthrpologist character dies of terror in a hospital emergency room. The story unfolds as his wife uncovers why he went mad.
Rosemary's Baby, with a very young Mia Farrow along with Frank Cassavetes, Ruth Gordon and Sidney Blackmer. Only one or two deaths as I recall and no blood or gore but an amazing use of mood and atmosphere to create a truly weird and horrifying story with a rather odd climax. I'm not sure that's actually Mia Farrow naked in the scene where Satan knocks up his bride but in 1967 it really stood out.