I agree and disagree with the statement that parents are the best teachers.
On the one hand, often there is a great bond between a child and a parents (especially if the parents are good), which allows for education and life lessons to be learned by the child. In this sense, parents are the best teachers. Moreover, since people learn by observation, the fact that parents and children live under one roof also allow children to learn many things about life through their parents. Hence, good parents leave a legacy to their children and can shape them in profound ways.
That said, not all parents are good. When this is the case, parents are not the best teachers. If a parent is abusive, dismissive, or immature, then parents can really harm a child. In these situations, other adults are better teachers.
In short, the potential is there for parents to be the best teachers, but parents need to make a concerted effort to capitalize on their time with their children.