Meh, Kitchener-Waterloo has a low vacancy rate, and a huge absorption rate for its size, but I'd be ecstatic to even see a "tall" (100m+) built downtown or uptown anytime in the near future (next 5 years).

I think St. John's would do very well if it could accomodate a couple of semi-highrise (50-100m) office buildings downtown, and that would be a reasonable thing to hope for. Worse case scenario, a lot of that demand could end up being accomodated through lowrise suburban office parks.

My bets are on 1) Toronto and 2) Calgary, for reasons that have been stated over and over again in this thread. They are realistically the only contenders for a building of 300m +, and even Calgary is pushing it for the near future.