Of all the secrets revealed in An Inspector Calls, which one is the most shocking to an audience and why?

The most shocking secret is probably the fact that Eric had made Eva Smith pregnant and that he had, out of desperation, stolen money from his father's business in order to support her. 

The reason why the revelation is so powerful is because firstly, Eric is the youngest member of the Birling family and that one would expect that he would, due to his youth, have had the least involvement with the unfortunate young woman. He appears, from the outset, to be quite upset about inspector Goole's assertions and, at one time, accuses him of having spoilt their little get-together. He is quite uncomfortable and promises to leave. He blurts out toward the end of act one, "Look here, I've had enough of this." He then informs the inspector that he had a little too much to drink and feels that he is in the way and wishes to go to bed.

Eric is also strongly critical of those he believes had done Eva Smith an injustice and continuously makes derisive comments in this regard. We do gradually learn throughout the play, though, that he lives a fairly secretive life. Both Sheila Birling and Gerald Croft  reveal that he had been steadily drinking more and more for the past two years - a revelation that shocks his mother.

What also makes the revelation about Eric's complicity in Eva's death surprising is the fact that she had been turned down for assistance when she approached the charity organization, chaired by Mrs. Birling, for financial assistance.  She rejected the destitute young woman's appeal for she thought her impudent and 'putting on airs.' She told Eva that the father of her unborn child was responsible and that she should approach him for help.

At this point, we have a growing suspicion about Eric's involvement, but we are not quite sure. Sheila displays a growing awareness of where the inspector is heading to with his interrogation of Mrs. Birling, which heightens not only the tension but also our expectation of what is to come next.

Even though the audience is prepared for the actual disclosure about Eric's role in the affair, it does not make it any less shocking when the truth is finally revealed: He had initially forced himself upon her and they later met again. Eva fell pregnant but did not want to marry Eric. It was then that he started stealing money from his father's firm to support her. When she discovered that he was providing her stolen money, she refused to accept any more. It was then that she decided to appeal to the charity for help. Her request was coldly rejected by the haughty Mrs. Birling.

In the end, Eva felt that she had no way out other than to kill herself. Her death, therefore, was as a direct result of Eric's actions and his mother's uncaring arrogance. Although all of the other characters had a role to play in her gradual descent into hopelessness, his actions were what eventually 'broke the camel's back.'