During the night: Place Ville-Marie with it's rotating beacon on the rooftop. It looks spectacular and it really is. You can notice it from 50 km.
1000 rue de la Gauchetiere is also beautiful at night.
Call me crazy but after Commerce Court North, my favorite Canadian skyscraper is First Canadian Place. It is so elegant and graceful, yet efficient and functional. It makes no pretensions or shows no delusion of grandeur. I just love it.