Anything I watch while on vacation will be driven by devices. If I'm going to watch a movie on my Kindle, I'm not picking a big-screen classic. Criterion is streaming lots of horror movies this October, including a bunch of Japanese movies from Ishirō Honda that were staples of my TV watching as a kid. The Mysterians came fairly early in his monster-movie career, and before he died, Honda said it was his favorite.
These posts will mostly be quickies... I'm typing this on my phone, for instance, which encourages brevity. The Mysterians is more science-fiction than monster movie, as an alien race from a destroyed planet comes to Earth ("in peace") looking for a place to relocate. No one takes them seriously... All they want is a small piece of land. They also want to teach the Earth people about the dangers of nuclear war (the reason their planet was destroyed). Like I say, sounds OK to me, but the reactions by the humans are completely hostile, with first the Japanese and then the entire Earth increasing their military response (and failing against the superior technology of the aliens).
Oh, there's one more thing: the aliens are largely radioactive, they have trouble making healthy babies, so they want to take a few female humans for mating purposes.
It's all predictably loony. It's fun watching these movies with subtitles. It's as if they quit being Saturday afternoon junk and become art films. The effects are good in The Mysterians. But I think it's a stretch to call it a clear classic outside of its genre.