In the book Catching Fire, there has been acts of rebellion and defiance. 

Katniss 

During her private session with the game makers, Katniss hung a dummy of former Head Gamemaker Seneca Crane. On page 236 it says that she wanted to "break through the smug veneer or those who used their brains to find amusing ways to kill (the tributes). To make (the gamemakers) realize that while (the tributes) we're vulnerable to the capitols cruelties, (the gamemakers are as well. It was defiant because she is sending a message to the gamemakers that they are not safe. She is reminding them that the last Head Gamemaker was killed by Snow and that they are disposable as well. 

Peeta 

During Peeta's private session with the gamemakers,  Peeta painted a picture of Rue. On page 240 it says (I just wanted to hold (the gamemakers) accountable for killing a little girl. This was an act of defiance because he is sending a message that it isn't ok to send little children into the games. 

Tributes 

After the interviews the stood in rows and the all held hands in unity. On page 258 the tributes all joined hands "up and down the rows the victors began to join hands. All 24 of (the victors) stand in one unbroken line." This is an act of defiance because they are uniting the districts. 

Cinna 

Cinna turns Katniss into a mockingjay on national T.V. On page 252 it says " Katniss started spinnign and the strange flames started burning away her dress and before she knew it she was the mockingjay." He turned the dress that Snow designed into a symbol of rebellion on national television. 

Conclusion 

I feel like president Snow found Cinna the most defiant because he turned Katniss in to a mockinjay. Then Cinna was killed infront of Katniss to hurt her right before she entered the arena.