In Lord of the Flies what does Ralph think is the most important thing that needs to be addressed on the island?

There is no single "right" answer to this question, which is really a task asking for an opinion and evidence to back that opinion up. Ralph is the central character of the novel and things are generally seen from Ralph's perspective. Ralph is the first character to appear in the novel, he also finds and blows the conch. Ralph is the chief, he wants them all to be rescued, he is hunted and he speaks to the naval officer at the end of the book. Jack is also very important because without him there would never have been any challenge to Ralph's leadership. It is Jack who hunts, Jack who leaves and sets up his own tribe, Jack who leaves an offering to the beast and Jack who is ultimately responsible for the killings of Simon and Piggy and for the hunting of Ralph. Good arguments could also be put forward for the importance of Piggy and of Simon.