Further to what @popping mentioned, AGT Mobility was a pioneering force (with government funding!) back when it was building out its initial network back in the 1980s, and that included very rural areas in northern Alberta. AT&T CANTEL (now Rogers) wasn't quite as early to the game, but they too were quite active in building out a network back then, and have very good coverage today, as a result.
I think it's safe to say any of these major carriers will have you covered. There will inevitably be a few holes here and there with any of them. OpenSignal.com used to have crowd-sourced coverage maps that you could view through your browser, but it seems they've moved all this functionality to their Apps. Definitely worthwhile to get the "OpenSignal - Speed Test & Maps" app... you can elect to contribute your phone's signals to the project's database or not, and the app itself is useful for seeing if you're in a deadspot or not, which carrier(s) have good signal, and even for finding free WiFi hotspots.
