According to me, Soul is the most beautiful thing in the world. Because the soul chooses to do, the good or the bad, the kind or the cruel. If the world was to become soul-less, there is going to be no place to live in harmony.
Besides this, the heart gets guided by the soul because the thoughts in our mind keep changing with time, but the deep rooted morality in our souls does not change. Hence, the soul is the most beautiful thing in the whole world.