I'm not reviewing the City Hall as a service. This is just the building itself.
City hall is a relatively new addition to the landscape of Old Montreal (built in the 1870s), but is just as impressive as the buildings built in the 1700s. I loved the details on the building and just the outside was a joy to look at, especially at night. The inside is less interesting, but the local government does daily guided tours (in French and English) detailing the history and government protocol (specifically the council chamber and associated rooms including a really neat balcony). The "hall of honour" is available to see anytime City Hall is open if you can't commit to the hour long tour. Best of all, it's free (tour and all), but it does have dedicated hours and reservations are required for the tours, so be sure to check the website for times and numbers. The closest Metro stop is the Champ-de-Mars, but is still a hike.
This would be of particular interest to government history or architecture fans.