"Worst" is subjective so a broad overview and several inputs would be appreciated
Worst can mean: The sin that brings about the greatest punishment (already mentioned); the sins that most distance one from God; sins that remove ones entire share from the world to come https://www.sefaria.org/Pirkei_Avot.3.11?lang=bi&with=all&lang2=en ; sins that push us furthest from the ultimate purpose of creation.
Feel free to add more definitions and bring more answers and debate which definition most finely encapsulates "worst"