I still like the clarity of Plato, where harmony is the basis of health, happiness, wellbeing, virtue, justice and the good. He describes harmony in the individual, in society and in the cosmos. For instance, when there is harmony in the parts of the soul (reason, appetite and passion) a person is virtuous and happy.

Obviously much more can be said -- especially considering that harmony is found in the tension of opposing forces -- but that's a remarkably enduring approach