What daily commercial flight is the oldest / has been running the longest?
According to the Georgia Encyclopedia one of the oldest flights operated by Delta was Love Field, Dallas to Birmingham, Alabama from 1929 up to 2000. Birmingham ist still served by Delta, but not from Love Field.

According to this the first flight from London to Schiphol was in May 1920. This route ist still in operation by KLM. But London at the time meant Croydon Airport.

On 27 May 1936 Aer Lingus made its first flight from Dublin to Bristol, while the route is also still in operation, the airfields changed. (wikipedia)

Take your pick.