How about introducing canonical-question to Stack Overflow in order to make it easier to identify them?
The general idea is to have either moderators designate individual canonicals and/or auto tag those questions which have been given most often as a possible duplicate. This would allow us to limit searching for duplicates inside this tag - which presumably has the best quality answers - instead of having to wade through the full list of duplicates of varying quality. It might also encourage more users to contribute to a canonical instead of spreading answers between the dups.
Regarding canonical-question being a meta tag: while that's true, I dont think it suffers from the same problem as best-practise, subjective or beginner mainly because it should be limited to either automatic application or by moderator decision. canonical-question, while a meta tag is a functional tag.
Regarding canonical-question should be canonical-answer: I think this does not represent how Stack Overflow currently works. Tags are for questions and I've seen questions hijacked by one good answer turn into a collection of other high quality answers, so I'd say it's the collection of question and answers that counts. Also, the question itself could be edited to be phrased in a more canonical sense. Example https://stackoverflow.com/posts/3577641/revisions