The worst audience behavior is....

Talking. I think it that because it distracts the people that are presenting. Also they might be someone else in the audience that is really interested in the show, and it is disturbed and distracted by the chating. Another good reason, is that the person that is preforming will feel bad. He works very hard and spends a lot of time practicing and preparing for the show, and when he sees that you are not paying attention, he will think that you didn't like and all the time and effort he had put in, were just a wast of time.