A sense of the unknown. Consider 'Fog. Whispers. Unseen danger.' The fog obscures vision, and the whispers suggest something that is not clearly defined. The unseen danger creates a feeling of unease because our imagination can run wild thinking about what that danger might be.
I'm not sure specifically which '20 best six - word stories' you refer to. There are many collections out there. Some might be about love like 'Met you. Loved you. Lost you.' Another could be a sad one 'Birthday. Alone. No calls. No cake.' And there could be a hopeful one such as 'Struggled. Fell. Got up. Kept going.'
The six scary stories could be filled with all kinds of terrifying elements like haunted houses, creepy creatures, and mysterious disappearances. Each story might have its own unique setting and source of horror, for example, one could be set in an old abandoned asylum where patients were once tortured, and now their ghosts roam the halls.
There could be a story where a small town is plagued by a series of mysterious deaths. People start to suspect that there is a serial killer on the loose, but as the story unfolds, it turns out to be something much more sinister, like an ancient evil that has been awakened. Another plot might involve a person who is haunted by their own doppelganger which slowly takes over their life, causing chaos and fear.