Notre-Dame Basilica is one grand Church right inside Montreal and is something you can't really miss when you visit Montreal. There simply isn't much to see here so I suggest you pay a visit.
I have seen a tons of churches, basilicas, and chapels around the world but I would say Notre-Dame is quite impressive although not as grand and elaborated as Duomo in Florence. There are two parts here; one is the Basilica and the back is the Chapel, You do have to pay $5 to enter, which I consider cheap. They also have English and French guided tour if you so desire.
They have the painted glass windows and bronze/gold looking architecture and it is very solemn and historic although some parts were restored after a major fire.