I think that you could do quite well with quotes that focus on power, leadership, and civilization. Those three themes are quite interrelated in this book, so a single quote could be applied to more than a single theme. I definitely recommend looking for quotes that focus on the boys, their government, the chief, and the conch. The following quote could be use in a variety of ways.
“Shut up,” said Ralph absently. He lifted the conch. “Seems to me we ought to have a chief to decide things.”
First, the quote has Ralph making a suggestion as to a government type. He recommends a government that has some type of head. It could be a monarchy, but it could also be a democracy similar to what the United States has with a president. The quote also works on a symbolic level for power and leadership, because Ralph says his suggestion while holding up the conch. The conch becomes a very powerful symbol of both power and leadership. Whoever controls the conch controls the boys. When the conch is finally destroyed, it is symbolic of the destruction of all rule and order.
Shortly after this quote comes another favorite quote of mine about power and leadership. This time the speaker is Jack.
“I ought to be chief,” said Jack with simple arrogance, “because I’m chapter chorister and head boy. I can sing C sharp.”
This is a fantastic quote in my opinion because it shows two things. First, it shows that Jack desires to be leader. He wants power. He will eventually seize it for himself. But this quote also shows just how ignorant Jack is about what qualities a good leader should have. He thinks his singing ability signifies his ability to organize and lead the other boys. It's laughable, but the fact that it does gain him some traction also shows how naive the rest of the boys are. Even their choice of Ralph as chief is suspect, as it is based on him being good-looking and the person who blew the conch.