Horror is a genre of fiction; it is not something you can apply to anything that is bad. For that we have the words "horrible" and, much less commonly and a little more specific, "horrific". Since each of those has three syllables, you would have to use them like this:
horrible
more horrible
most horrible
The best way to apply this to a horror movie would be to kind of get around it and say something like "the movie that is the most focused on being a horror film".
...As mentioned in the comments, "horror", "horrible", "horrific", and "horrifying" all have different meanings, with "horrific" and "horrifying" being particularly close in definition.