I imagine Vancouver is the city that would most likely be able to support an NBA team assuming they had a good ownership situation. Its population large and wealthy enough to support it, and probably would get a lot of interest from the Asian community which generally enjoys basketball compared to the other major North American sports. Also in recent years many college stars and a few NBA players have come from BC, likely as response to a generation that remembers the Grizzlies and inspired them to play the game. Montreal has the population, but I am not sure there would be enough interest in the NBA. It seems more like a baseball town, and is most likely the MLB's highest priority (I could be wrong though) for expansion/ relocation. Calgary, Edmonton, Winnipeg, Ottawa, etc. are too small to support an NBA franchise on top of their NHL and CFL franchises, and even if they were large enough, would there be enough interest? I doubt it.
When considering a city's demographics, you need to consider their wealth on top of their historical taste in sports. An example would be that Winnipeg has a large Filipino community which is basketball crazy, but most of them don't make enough money to be able to afford season tickets, or go to more than a couple of games a year to a major North American sports league. So a city can be very large, and multi-culturally diverse, but if their income is not high enough a pro sports franchise will still fail, even if the population is very interested.
Overall I think the NBA would be looking at Seattle before any Canadian city for NBA expansion/ relocation.