I'm not sure if Canadians have the best English pronunciation as a rule. As for Americans speaking the best English, I doubt that is the case. All I can say for sure is that native speakers of English tend to speak much better English than non-native speakers of English do. As a result, many, many educational institutions in China only accept applications from native speakers of English for teaching vacancies requiring the English language to be spoken and/or written in class. You see, almost all non-native speakers of English, even most of those with post-graduate academic qualifications, cannot speak and write English nearly as well as well-educated native speakers of English can. One would have thought that what I have stated is common sense, but clearly it isn't to some foreigners teaching in China, and so needs to be posted here in black and white!