Northwest hands down. It will get you plenty of places domestically and you'll get upgraded a lot automatically. Also, AS is a strong partner of theirs as is CO. That will mean a ton of upgrades. CO has a good route structure for south America. Your best Calgary option is AS. There is no non-stop to Toronto, so you'll just fly through MSP. Stay away from UA and AA. Their domestic products aren't that great, their lounges are terrible and you'll have a lot more benefits in NW.