You are quite correct that the Canadians have the best English accent- clear and articulate. In the United States I worked with a Canadian ESL teacher, and his accent is what impressed me the most. Last week I played for my Chinese students differrent radio shows from English speaking countries around the world. Their task was to choose an accent they thought was the clearest one to practice repeating the words. The last thing I told them was that after I am gone they would be luky to get a Canadian teacher.
However, not all American English is bad. The English on radio stations like National Public Radio have excellent accents from which students can learn. That said, Texas Twang and the accent from New England would not be advisable to practice imitating.