As has been noted, the best views of Montreal are to be had from Mount Royal, and for good reason: not only does it look out over all of downtown and the St. Lawrence, but its peak is at about the same height as many of Montreal's bigger office buildings. When you go up to Kondiaronk Belvedere, the main lookout area, don't miss the little brass sights on the edge, which point out all the major landmarks.
Another nice thing about Mount Royal is its accessibility; the walk up the winding road isn't too taxing, but if you want to get up faster, and don't mind a bit of a workout, there are a series of stairs that go right up the slope. It's an excellent place to go for a walk if you live off Parc or in the McGill ghetto, all year round.