Ottawa we can pretty safely drop from the list -- as a planned city populated mostly by government bureaucrats, it's got a few good museums but very little else to interest the casual visitor, and it's quite hard to get around without a car.
Montreal vs Toronto is a more interesting showdown. Personally, I'd cast my vote for Montreal: it feels a bit smaller and more approachable, with an incredibly vibrant cultural scene, and the presence of French everywhere gives it quite a unique flavor for a North American city. However, if you're keen on something that's less "European" and more like New York or Chicago, then Toronto may fit the bill better.
All that said, my favorite big city in Canada is actually Vancouver, which also has the most scenic location by a long shot and arguably the best climate as well.