Some of us use the term bad movies to mean, simply, films that emerge from a supposedly lowbrow genre, or films that are stylized in the manner we tend to label camp. (Road House from 1989 is this kind of bad movie, and is very good at being one.)
There is no single "most important" element as all film components story, characters, acting, direction, production quality, pacing, emotional impact, relatability, and originality contribute to its overall quality. However, many argue that a compelling story is at the heart of a great movie.