Formally, the idea of the good is the end in itself that is sought for its own sake. When we begin to identify what this end is, the good quickly becomes worldview relative. This means that what a given person believes to be the good assumes a number of other beliefs which form a way of thinking about the world and interpreting experiences.
Discussions about what is good will quickly reach an impasse if this greater worldview context is not taken into consideration. This is partly what Alasdiar MacIntyre is getting at in his book "Whose Justice? Which Rationality?" (for a review of MacIntyre's work here is a link to an article in "First Things" by Stanley Hauerwas). What a person believes about the good contains assumptions about what it means to be a human, the reality in which humans live, and the basis of authority for justifying these beliefs.
I'd like to consider a couple examples here, with the ability to add more in the future. There has been an emphasis in the Modern West on the goods of this world. Sometimes this becomes a denial that anything exists besides the material world. Secular Humanism is the worldview which claims that only the material world exists. Human nature is only physical, and human welfare only for this life. Secular humanism defines this welfare in terms of happiness, while making the standard qualification that happiness cannot be sought in a way that harms anyone else.
Secular Humanism claims experience as its highest authority. However, this means experience interpreted in light of the basic beliefs that only the material world exists, and that humans are completely reducible to the body. Sometimes called "naturalism," this interpretive lens affects how experience is understood and how scientific claims are extended into non-observable areas, such as origins.
For instance, one cannot prove from experience that only the material world exists, or that humans are simply their body and nothing more. We can call these "pre-empirical" in that such beliefs are used to interpret the empirical data, but are not themselves derived from the empirical data.
Secular Humanists argue that the good is the increase of happiness in this life. With respect to this view of the good, Secular Humanism faces the same dilemma as their ancient counterparts, the Epicureans and the Stoics. Caught in materialist assumptions, and agreeing that the good is happiness, these groups argued about whether the best life is the life of pleasure or virtue. Given materialist assumptions I suspect the Epicureans have an edge, although I also argue that this dilemma cannot be rationally resolved if materialism is assumed to be true. Today this is a dilemma between ethical egoists and utilitarians.
In contrast to Secular Humanism is something I will call Western Dualism. This view, found in Plato and Platonic interpretations of Judaism and Christianity, assumes that both the material world and the soul exist. These are not created by God, rather God is the power that forms the world. The material world and the soul are co-eternal with God. Socrates discusses some of this in Book X of The Republic. In Plato, the soul is eternal but forgets this truth when born into a body.
In this view, the highest good is a blessed afterlife once a person is no longer restricted by the contraints and pains of the physical body. The body is said to be limited, and this limitations is interpreted as the source of suffering and evil. Appetites led to moral evil. In contrast, once the soul is released from the body and the chains of the senses, it can gaze directly on the highest reality (the beatific vision).
In the West, these two views have become a kind of false antinomy both in popular thought and in philosophy. The unproven assertions of Plato led to the "Academic Skepticism" of his school, the Academy. This, in turn, led to the more practical views held by the Epicureans and Stoics, who were also locked in skepticism.
In both cases, Secular Human and Platonic Dualism, the truth of basic beliefs is assumed. This is called fideism (blind belief). The rest of the worldview, including beliefs about the good, is affected by this skepticism and rather than give rational arguments there is a turn to more personal kinds of proof. An example of this can be found in Cicero's "On Moral Ends", in which Cicero, the Academic Skeptic, provides us with a dialogue about the best life.
The implication is that if we are going to make progress in knowing the good, we must first make progress with the basic beliefs assumed by the good.