Vancouver is right next to Seattle, and wouldn't diversify Amazon in terms of time zones or meterological/geological catastrophes. I don't know enough to say for sure, but I think Amazon has a preference for English-speaking cities which would indicate against Montreal.

Honestly, I think Toronto is a huge dark horse. It's an incredibly attractive location to relocate candidates to and isn't subject to American visa laws. I think the main reason smaller Canadian cities didn't make the cut is because Amazon doesn't really want to go that far north. Toronto is already south of Seattle, for instance.