Since there are unlimited numbers of Minecraft seeds, no one knows what the scariest Minecraft seed is. That fact does not seem to stop people posting articles claiming to know the scariest Minecraft seeds, though. I guess it’s become a colloquial expression to exaggerate and/or not understand what a superlative is.
Anyway, it seems to me that these are pretty scary, especially if you are in survival mode:
Relentlous
12647
12345
Found at: https://www.gameskinny.com/w4lyb/the-5-creepiest-minecraft-seeds-to-scare-you
Of course, one of the things about Minecraft is that updates to it, or possibly to Java, or playing on a different platform (computer, phone, console) may mean the world generates differently for the same seed, possibly changing what a seed world is like entirely.