I haven't researched it yet, but I was once told that sin originally meant ignorance. During the bastardizations of the many Sacred Texts the term was misinterpreted, or so I was told. Anyway to answer your question.
I 'feel' that the ultimate sin is rebellion. Rebellion to our inner God-self/ves.
We are all born with the capacity of accessing good and evil. As we spiritually and mentally mature we gain more insight which enables us to see more solid blacks and whites in the fields of grays. When this clarity comes into focus we begin to make more solid and absolute choices. When we go out of our way to access the evil for whatever reason we 'sin.'
I say that it's rebellion. I hope I explained my rational clear enough.