Every town has a main street. When I was growing up in Hamilton, Main Street was King Street (there was a Main Street...it ran parallel to King, but all the action was at King and James.) In Montreal, Main Street would probably be Ste. Catherine, or perhaps St. Laurent, which is actually nicknamed The Main.
But away from the larger context of the city, within my neighbourhood, there is St-Viateur. It's home to St. Michael's and St. Anthony's Church, which defines the local skyline every bit as much as St. Paul's Cathedral once did London's. There is Arahova's, Open Da Night (officially known as Café Olypico) and some shops. It's also home to some amazing street parties in the summer.
Here is St-Viateur, on a gorgeous morning a few days ago.