I am not fully agree with that parents are the best teachers.
Admittedly , parents have a great influence on their children. Form an infant to a child , people spend most time with their parents sitting together watching TV, talking about funny things happened around the neighborhood ,driving out for a picnic, etc. In a child's view, parents are absolutely right and very powerful. Consequently , children tend to imitate their parents behavior in daily life.
That's how the first and may be the most important education starts. It is parents who teach their children learn the rules of the society and the relationship between human beings. Remember you father told you to respect the old and to be truthful to your friends? According to a research, ninety percent of people reported knowing the principle of being a human and the basic of the world from their parents.
However, influence could be good and bad. Some parents really set bad examples for their children. There is a high rate of divorce in many countries and divorce can really mislead children's understanding to people's relationship. Form a survey , eighty percent of teenagers come from divorce families tend to behave more aggressively and unfriendly than those who come from normal families. What's worse, some parents smoke heavily , drink regularly or even behave violently in front of their children are not only bad for their physical health but also make damage to their metal health.
To sum up, I think parents can either have a positive or a negative influence on their children. So it won't be rational say all parents are good teachers even if most of them are doing a good job. It still depends on their morality and personality.