If the question was merely "which airline has the best lounge?", I am sure we would get a variety of answers.

But since North America is larger than Europe (duh!), a better coverage in N.A. is more important to an NA flyer than a worldwide coverage. Elite members (in S.A. or 1W) get international access unless they are in N.A. (where they are most likely members of N.A. programs that do not count N.A. flights as international), the choices left the US and Canadian airlines' lounge programs (I am not familiar with Mexican airlines).

And Ken H. has pretty much explained the big difference with Maple Leaf and Empress is the lack of reciprocity between AC and UA.

CP/AA has the most travelled destinations of a North American flyer covered (NA, Caribbean, Asia, Western Europe), coupled with its low cost, I'd give it a two flying thumbs up.