Threads is the most harrowing movie by far. Remember watching it in college and not being able to get out of bed the next day because I was so fucking depressed.
I remember watching it when it originally aired when I was 13, and having to talk about it at school the next day - we were in the middle of an assignment on the threat of nuclear war at the time, it was tacked into our english language class as I recall (tbf, I can't really think of anything else, other than possibly Physics, where the topic would have been better suited, and Physics could probably only have carried a very dry discussion of it).