Every community, inevitably, has a value or set of values that it considers fundamental, some basic good which positions every other claim to goodness. For most of the modern liberal democracies, for example, freedom and equality play this determining role in the moral discourse.
In Communist societies, economic justice, construed as the elimination of the class structure, would provide such a foundation. In the context of German National Socialism, the defense of the Fatherland and the will of the Führer anchored the moral system, however corrupt.
There is a rather simple means of identifying this ultimate value: in regard to any particular moral or political act, keep asking the question, “Why is this being done?” until you come to the point where you find yourself saying, “Well, because that’s just a good thing.” The “just a good thing” is the value that your society or culture considers non-negotiable and which in turn determines all subordinate values.