From my own limited experience I have found that the greatest degree of inner tranquillity comes from the development of love and compassion.
The more we care for the happiness of others, the greater our own sense of well-being becomes. Cultivating a close, warm-hearted feeling for others automatically puts the mind at ease. This helps remove whatever fears or insecurities we may have and gives us the strength to cope with any obstacles we encounter. It is the ultimate source of success in life.