I found this website which has links to panoramic views of several of Montreal's most beautiful churches, and therefore to some of Canada's most beautiful churches. In all seriousness, Toronto's most beautiful church, St Paul's, wouldn't stand out as anything special in Montreal.
The views are quite special. Here is the exquisite Basilique de Notre Dame, regarded by many as the most beautiful church in Canada. The link didn't actually come from the original site, so I linked to another.
Here is Marie Reine Du Monde Cathedral. The Cathedral is actually a 1/3 scale replica of St Peter's in Rome, complete with a scale version of the great baldacchino. The rest of the decorations are not the same, but it is still worth seeing. It was built in what was at the time the English Protestant part of Montreal. The bishop, Ignace Bourget, who was an ultramontanist, was sending the Protestants a message as to who ran the city.
The Chapel of Notre Dame de Bon Secours. This is Montreal's oldest chapel, built in 1771 on the site of an even earlier chapel, founded by St Magarite Bourgeoys. There is a museum underneath showing some of the archaeological site. This chapel was close to the old port and was frequently visited by sailors, which is why in the panorama you sometimes see little wooden boats hanging from the ceiling. These were offerings left by sailors who were either praying for a safe voyage, or were giving thanks for a safe return.
Here is St Joseph's Oratory, founded by the recently canonized Br Andre. The panorama is of the exterior. I've been inside, and while the crypt chapel is nice, the basilica above is... not so nice. The exterior is really the best part of it.
Here is my favourite church in Montreal, St Patrick's, built for the famine Irish in 1848. It is a little more restrained than Notre Dame, but also with a far lighter interior. It is also quieter, with far fewer tourists.
As a bonus, here is the first Italian church built in Montreal: Madonna Della Difesa. See if you can spot Mussolini in heroic equestrian pose in one of Guido Nincheri's frescoes. There's a funny story behind that.
So there you go. Checkest thou it out.
Update: My wife pointed out that I passed up a golden opportunity for one of my common lines when I wrote about the Montreal Cathedral. I have pointed out in the past that churches were often designed with input from the parish priests, who seemed to be given a carte blanche on building their own churches. What often happened was that a priest would approach an architect and say something on the lines of "There was this church that I saw when I travelled to X, and I liked it. Could you design a church along those lines?" My joke is that, had I been a priest of that era, I would have said to the architect "There was this chuch I saw in my travels to Rome. It went by the name of St. Peter's. Build me another one." I did not realize, at the time I made that joke, that someone had done exactly that. I am left with the uncomfortable truth that my faith is not only weirder than I imagine, or weirder than I can imagine, it is even weirder than I can parody.
Of interest: First post in an earlier series I did on beautiful churches in English Canada.