I would say that from what we have seen from the canonical episodes and judging from the scenes that revealed large ship congregations, that the single most widely produced capital-grade Federation starship was the MIRANDA CLASS. This ship is easily augmented in different configurations, it is easy to upgrade with current level weaponry, computers and hardware and it's internal spaceframe was designed to be compartmentalized. It uses less material to construct than the old Constitution-class refits, yet carries more weaponry and support craft housing. This ship was seen from the Wrath of Khan all the way to the very last episode of Voyager, which would have put it in service for well over 100 years. Clearly, this class has remained a durable and versatile ship and their production or at least their components production should continue well into the 25th century. This ship was intended to be replaced by the Sabre-Class fifth generation starship first seen in ST First Contact.