Any horror fan has seen a movie or two that you'd never recommend to others, and you might even warn them away from wasting their time. There are movies that rehash a well-known plot, only worse. There are low-budget effects that aren't at all believable. Some get produced with poor acting, slow pacing, or distracting continuity mistakes. In fact, there are so many ways to make a "bad" movie that they give you an appreciation for the ones that turn out decent. That's about the best thing you can say about the worst of the worst.
Lists of the "best" movies have an upper limit, but when we talk about the "worst" horror movies, it's possible to consider movies that closed quickly after opening, those that exist only on bootleg tape, and even homemade YouTube videos. In this list of the worst horror films of all time, the floor was established by requiring that all entries have at least 20 reviews on Rotten Tomatoes. That means you've probably seen quite a few of them, and you can compare your reaction to what other people thought. Mental Floss listed 25 horror films that scored abysmally low, and takes a closer look at the six horror films that scored 0% on the Tomatometer.