I watched Threads for the first time recently and it really did ruin the rest of my day. I still think about it regularly. It influenced my thinking on the threat of nuclear annihilation a lot.
If you don't feel like watching the whole film (and you definitely should, the first third is all lead-up and it's masterfully done - and the aftermath part is the most believable post-apocalypse I've ever seen in a movie) you should at least watch the bombing scene:
The attack scene from the American version, The Day After (1983), is even more harrowing in my opinion. I looked up how the special effects for it were created, even though looking it up usually ruins a movie for me (I have stopped looking up the special effects for movies). The attack/fallout scene from How I Live Now (2013) is also eerie. These are not snuff scenes, that's not what they are about. The movies were made to make people think about the entirety of circumstances surrounding such an attack.
How I live now, while not a particularly good movie, really sat with me. The lack of understanding of what's happening around them, as the government has crumbled there is no civil order to communicate with citizens. The parts where they traverse the abandoned motorways, encountering highwaymen and rape gangs... it all left a really weird feeling.
If you don't feel like watching the whole film (and you definitely should, the first third is all lead-up and it's masterfully done - and the aftermath part is the most believable post-apocalypse I've ever seen in a movie) you should at least watch the bombing scene:
https://www.youtube.com/watch?v=MrHoMSRZOS4