For me it was Event Horizon, that was scary as hell when I was a kid, I had nightmares for months.
two movies came to me pretty vividly,
Spirited Away (2001), no-face is pretty scary even now, but the scene that disturbs me is at the beginning when Chihiro came back and find her parents have turned into pigsβ¦
MirrorMask (2005), fittingly I watched this when I was sick with high fever and for so long I thought this movie was a fever dream, it haunted me for days until my fever subsides. I donβt remember anything about the plot, just that the atmosphere and aesthetic are nauseating