The most unsettling film I've ever seen is "Us" (2019). Written and directed by Jordan Peele, the film centers on a family who's seemingly normal beach holiday is disrupted by the arrival of doppelgangers of themselves. The ensuing events are filled with tension and horror, as the family must battle against their own dark reflections for survival. The film's unsettling imagery and narrative twists make it an effective and disturbing experience that stay with the viewer long after the credits roll.