My guess would be American or Northwest. Northwest has got a good route network to central and western Canada via their Minneapolis hub. American has got a good amount of flights from New York, Boston, Chicago, and Dallas to major cities throughout Canada.
AA I know flies to YHZ, YQB, YUL, YOW, YYZ, YYC, and YVR, and I'm not sure, but they may fly to YWG and YEG as well. Northwest, I'm not so sure about, but I know they fly to YYZ, YWG, and YYC. I'm sure there are more flights.
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