For horror fiction, I’d say games or books, depending on what you’re going for. Visual, you could go with film, but games are more interactive; otherwise, the same thing (visual). Psychological, like getting inside someone’s head, like Pet Sematary does, I’d say the book is the superior option — or the audiobook narrated by Michael C. Hall (Dexter).

For real life/true horror, life experience. I’d say there’s not much scarier than not knowing if you’ll survive the night because you live with a psychotic partner (or parent). For the partner, watching the violent partner drink and trying to control the mood with the TV or music trying to keep the atmosphere fun or at least safe, and them teetering on a violent outbreak, or being a child witnessing this and huddling in a closet or under a bed having to listen to one parent being beaten and hoping the violent one doesn’t come for you. Or they pull one of your siblings out and you hope you’re not next. Stuff like that, I think is worse than any fictional horror. There was a video game, Alien Isolation, that tried to put you in a situation like that, but it was just a video game, you could pause it and get up and take a walk (or, if you couldn’t pause it (the Mass Effect trilogy was great at not respecting your time, despite being amazing otherwise), just turn it off). In real life you can only wait it out. Or make a run for a window or the door and hope you can get help or something.