The Musée des Beaux-Arts is made up of a number of pavilions on Sherbrooke West, both neo-classical and modern, which reflects the wide range of its permanent collection. There's usually something for everyone.
They have a lot of fascinating special exhibitions. When I first visited Montreal as a kid, I saw a fantastic show of cars here--they controversially moved all the art out and filled the galleries with cool, often one-of-a-kind cars from automotive history. More recently, I saw an exhibit of Jacques Callot prints (as well as many exquisite, original plates), which, coincidentally, occurred a few months after I did a French history term paper on his "Miseries of War" series.