According to the inventors of sin, sin is a crime or offense against God. If there is no deity, sin is impossible to commit. Also, they say all sin is equal: you are equally condemned for committing murder or for looking at someone lustfully. So this question, as worded, is deceptively hard to answer.

If you're asking "what's the most awesome thing you've done that people would consider a sin" well, I'd probably add to the list of people who answered with atheism, as questioning and searching for the truth is taboo in Abrahamic societies.

If you're asking "what's the most awful thing you've done?" Well, it might be somewhat tame, but I wake up in a cold sweat sometimes when I think about it. Imagine a 13-year-old kid confident of having the answers to someone's questions, when a life is on the line.