"Scariest" isn't one thing. Some horror games get you with sudden jump-scares, others with slow dread, others by making you run for your life. The best one for you depends on which kind of fear you actually enjoy. Here's the genre sorted by how it gets you.
Scariest for chase panic
The pure adrenaline of being hunted. Doors is the modern king, where entities like Rush turn a quiet hallway into a sprint; Survive the Killer and Flee the Facility put a real player on your tail; and Piggy built a whole phenomenon on the chase. Best played with friends screaming on voice chat.
Scariest for slow dread
If you prefer atmosphere over adrenaline, Alone in a Dark House is a slow-burn investigation that gets under your skin, and The Mimic uses Japanese folklore and maze-like maps to build sustained unease. These don't chase you — they sit with you.
Scariest for the unknown
Some horror is about not knowing the rules. Apeirophobia traps you in the Backrooms, 3008 makes an endless furniture store turn hostile at night, and Specter has you identifying a ghost before it identifies you. The fear is in the figuring-out.
Scariest with friends
Horror is almost always better in a group, and these are built for it. Doors, Specter, and 3008 all reward — and punish — teamwork, which somehow makes them scarier, not safer.
How to pick your scare
Be honest about what you can handle. If jump-scares ruin your night, lean toward the slow-dread picks. If you love the rush, go for the chase games. The full horror genre page lists every horror game in the directory, and each one tells you what kind of scary it is before you commit.