Threads isn't a horror movie in the conventional sense (no zombies or monsters), but it’s absolutely one of the most horrifying films ever made. It’s a hyper-realistic depiction of how a nuclear attack would devastate a city (Sheffield) and the long-term collapse of society afterward.
Sadly the young girl who played Ruth's daughter near the end of the film lost her life, just a few years after. She was only in her 20s IIRC :-(