Personally I think Vancouver BC is the best Canadian spot because of the significant Asian population and international influence, as well as not directly encroaching on Toronto's core fanbase in Quebec/Ontario. When comparing to the western US expansions candidates, Salt Lake would be like Denver on steroids and Portland is too small and too close to Seattle. The nostalgic side of me would really love the Expos to come back though.
Having seen some Canadians discuss this, the two primary issues are not surprisingly (1) ownership and (2) stadium. The Blue Jays are owned by the albatross that is Rodgers and there are only a couple other potential players with big enough pockets in all of Canada. Montreal would be starting from scratch on a stadium but at least in theory they could play at the big O in the interim. Vancouver is interesting because the Whitecaps (MLS) are trying to leave the downtown stadium (BC Place) which would potentially open it up for a renovation into a baseball stadium. That would still be costly but would be an excellent location.
There's no way in a million years that they expand to Alberta. SLC has stronger ownership interest with deeper pockets.