There is no doubt that parents, more or less, have influence on their children. However, hardly can one says that all the effects are positive. So when it comes to the argument that whether parents are the best teacher of children, i should say no.   At first, i should acknowledge that parents are the first teacher of their children. As everybody knows, children, especially at very age before going to school, always treat their parents as examples, even as idols. So parents are somebody who give their children the first lesson. 
However, as children grow up, they slowly find that their parents cannot help them with everything. For example, when children meet some problems in the homework, seldom exist parents who can solve all the problems in every course. Taking the social development into consideration, what parents learned in their ages is totally different form what children study today. So at the level of study, parents cannot be the best teacher.
When it comes to the level of life, i should say parents cannot be the best teacher neither. No one can be perfect. Parents' merits, of course, can have an positive effect on their children. But at the same time, it is unavoidable for children to continue their parents' flaws without other people's correction. What is more, Parent always like to push their children to achieve some goals which they did not achieve in their days. Maybe they think, according to their help, this way can be easier for their children to continue. Actually, Parents' dream may not be children's. So sometimes, parents' love turns out to be harmful to children's growth.  Finally, nowadays, parents are very busy with their work and do not have so much time on their children even if they want to. So, all in all, they cannot be the best teacher of children.