By far, the answer is YUL-YYZ for Canada.
Air Canada fills widebodies like A330s, A340s and 767s every hour
from dawn to dusk on weekdays! The journey is only one hour long!
In second place is YYZ-YVR. Air Canada and Canadian Airlines fill A320s, 767s, A330s, A340s and 747s on an hourly basis (I think) during the daytime.
Internationally (not counting the States), the heaviest route is YYZ-London, with about ten flights a day on Air Canada (A330s, A340s, 747s, 767s), British Airways (747s, 777s), and various charters, including Canada 3000 (A330s?), Air Transat (A330s or L1011s?), Royal Airlines (A330s), etc. Only Air Canada and British Airways fly to LHR. The others fly to LGW.
Pretty good for a country with only 30 million people!