An act of defiance is when a person goes against authority. In the book Catching Fire, we see many acts of defiance or rebellion.
   Katniss also does acts of defiance. She showed her skill as tying knots in the private session. When she was tying knots, she put a manikin in it and hung the manikin and put Seneca crane on it. this would be considered an act of defiance because she's saying that she's killed the other game maker  and she will that game maker
    Peeta does some acts of defiance too. He painted a picture of Rue the floor of the private session. This would be considered an act of defiance because he's saying that he will avenge Rue by going against authority.
    In the interview, the tributes said that President Snow doesn't have much power because if  he did, he would stop the Hunger Games. They held hands to show that their not enemies and that their united. This would be considered rebellious acts because they are turning people against Snow and also showing they are not enemies.
    During Katniss's interview it was reveled that Cinna turned her wedding dress into a Mocking jay when she spun around. This would be considered an act of defiance because it's a symbol of the rebellion.
    Out of them all I think that what Cinna did was the most defiant because he made Katniss's wedding dress into a symbol of rebellion.