What was the most important rule in William Golding's Lord of the Flies?

In the novel, the most important thing for the boys to do is to keep the signal fire lit because it is their only hope for being rescued. Ralph, as chief, establishes the rule early on that the fire must be tended constantly, and Jack volunteers his hunters to keep the fire going and to act as lookouts. However, Jack fails in this role, deliberately drawing Samneric away from the fire in order to help the hunters. Just at that time, a ship passes the island; had the fire been going, the boys might have been rescued. Ralph is furious and calls a meeting to reiterate the rules. He emphasizes the role of the signal fire several times. First, he says:

"Look at us! How many are we? And yet we can't keep a fire going to make smoke. Don't you understand? Can't you see we ought to--ought to die before we let the fire go out?"

Ralph makes the fire a matter of life and death--which it is. The hunters laugh self-consciously a this, and Ralph says, "I tell you the smoke is more important than the pig, however often you kill one. Do all of you see?" He then sums up, "We've got to make smoke up there--or die." 

Later, when "the beast" is believed to occupy the mountain top, Ralph becomes very discouraged and says, "So we can't have a signal fire. ... We're beaten." After the death of Simon, Ralph starts to lose focus and Piggy is the one who has to keep reminding him and the others of the importance of keeping the fire lit. Ironically, it is Jack's fire, meant to smoke out Ralph so they can murder him, that results in the boys' rescue--just in the nick of time.