Another one for Montreal:
Place Bonaventure
Once there was a mall in that monstrosity. Beginning in the late 80s, it started a severe decline with the development of several other downtown malls to the north (Eaton's Centre, Place Montreal Trust, Place Mont-Royale, etc.)
The monster under construction (1965-1967); at the time, the world's largest single building in square footage:
Here is a snapshot of the former retail area:
Today, there is less than 22K of retail, versus 197K before the renovations.
Today:
Other dead downtown malls in Montreal include Place du Canada, and Place Victoria.