When comparing one thing with all others, we use the superlative form of an adjective.
Bad → positive degree
Worse → comparative degree (used to compare two things)
Worst → superlative degree (used to compare more than two things or to indicate the highest intensity)
Badly → adverb, so it cannot describe the noun “movie”
Since the sentence refers to all the movies the speaker has ever seen, we need the superlative form: “worst.”
