Martyn Lloyd-Jones on priority of private prayer:
Ultimately, therefore, a man discovers the real condition of his spiritual life when he examines himself in private, when he is alone with God… The real danger for a man who leads congregation in a public act of prayer is that he may be addressing the congregation rather than God. But when we are alone in the presence of God that is no longer possible. And have we not all known what it is to find that, somehow, we have less to say to God when we are alone than when we are in the presence of others? It should not be so; but it often is. So that it is when we have left the realm of activities and outward dealings with other people, and are alone with God, that we really know where we stand in a spiritual sense. It is not only the highest activity of the soul, it is the ultimate test of our true spiritual condition [Studies in the Sermon on the Mount, 2 volumes (Grand Rapids: Wm. B. Eerdmans, 1993), 2:46].