I'd give Montreal a vote, I think the city offers the most concise statement on all things Canadian, from the existence of the French and English side by side, the remarkable cultural diversity, a nice amount of old structures and history along the river, and honestly, fun.

The dining is world-class, the feel is European and North American at the same time, and it's easy to get around by Metro (a tourist attraction in and of itself).

I found Toronto to be a big, and ugly city. Been there a few times, tried to like it, and just couldn't really enjoy it so much. The view of downtown from the Islands is worth it, the CN tower, but it's somewhat lacking in character.

If you're willing to deal with the long flight, Vancouver should also be an option. The natural beauty is incredible, with good weather (right now anyways), and a decidedly hippie meets Asia kind of flavor to it. Montreal and Vancouver are a toss-up for my favorite Canadian cities (that I've visited).