In Montreal, it most definitively is
Auberge Le Saint-Gabriel, established in 1754. The building itself was build in 1688.
Source
Other notable landmarks:
Montréal Pool Room (1912)
Chez Schwartz’s Delicatessen (1928)
Orange Julep (1932)
Moishes Steakhouse (1938)
Wilensky’s Light Lunch (1932):
Pizzeria Napoletana (since 1948 - oldest pizzeria in the city)
Beauty’s Luncheonette (1942)