Is eating beef greatest sin. Not worshipping God is greatest sin. Or some thing else.
Papa and Punya is not defined in absolute term. Gita in Ch.4 solka 17 says गहना कर्मणोगति (The intricacies of action are very hard to understand.). As an example read the story of Mandavya in Mahabharat (here). Even the great Rishi is punished for a small trivial action. Vyasa knows the best. He says: "Ashtadasa Puraneshu Vyasena Vachanam Dwayam, Paropakaraya Punyaya – Papaya Para Peedanam". Meaning: Two important sentence of Vyasa which can be the essence of 18 puranas are “helping others is punya and hurting others is papa”.