The superlative form of the adjective "bad" is "worst." While regular adjectives add "-est" (like "tall" to "tallest"), "bad" is irregular, skipping "baddest" for "worst."
Use it to rank something as the most bad among three or more—e.g., “This storm was bad, the last was worse, but today’s is the worst.” It’s a quirky rule, but easy to grasp!
Further reading: What is the comparative form of the adjective “bad”?