What is Kino's most valuable possession and why?
It depends on what point in the story you are referring to. At the beginning he values his family, especially his son and his canoe. As the story progresses the pearl becomes his most valued possession. This is seen when the value of the pearl becomes exaggerated in his mind and he sees it as his ticket to a new more respectable life. Eventually however, he places it before the welfare and safety of his family, his home and even his own life.
Ultimately, the pearl becomes the all-consuming object of Kino's life. Like for the character Gollum from Lord of the Rings, who originally possessed the ring, the pearl warps Kino's mind turning him from a simple, indigenous man to an aggressive, machine-like entity.