What is your home city? What about your common business destinations and desired award destinations?

Approximately how many paid miles will you fly in a given year? Less than 25k, 25k to 50k, or over 50k?

The legacy or "big alliance" carriers like United, American, Delta, etc. make travel quite difficult for people who don't fly at least 25,000 miles in a year. A non-legacy, more standalone carrier like Southwest tends to be better for <25k travelers.

In your case, it also might depend on how often you fly to Canada and how deep into Canada you go. A Southwest flier can get to Vancouver or Toronto easily enough (and maybe a handful of other near-border places I'm not thinking of), but to get to more far-flung Canadian towns you'd probably want to align with UA, US, or CO for maximum options since Air Canada is a partner.

If Mexico is a leisure destination for you, someone like Frontier could work. If it's a frequent business destination, then I know AA is strong and I think CO is too. I'm a United guy but always end up on AA when I go to Mexico.