Thor-class main battle frigate

The Thor-class main battle frigate, most often called simply as the Thor class, is one of the most widely produced ship models in galactic history. Introduced in the early 2400s, it has remained the backbone of the Terran Navy ever since. 
First introduced as a successor to the Ares class, the class which had brought victory to the Federation during the First Contact War, the Thor class was introduced in 2421, with prototype construction having commenced as early as 2398. The Thor class was envisioned as a multi-purpose ship, which was capable of being mass-produced. Upon its introduction, it boasted the most powerful shields of any Terran ship. It was also very cheap to manufacture, and mass production began in the 2440s. The Federation wanted to be ready for a renewed conflict with the Ur-Han, which at the time seemed inevitable. 
During the Great War, the Thor class was the one that spearheaded every assault and deflected every attack. However, even with the extensive upgrades it had received, it began to show its age. The 200 years which had passed between its design and the war had seen tremendous advancements. These were most noticeable in weaponry, whilst the Thor's shields and armour remained very similar to their original designs. The energy output of the generator, hard to increase, limits both the shield and FTL drive outputs. The FTL, pushed to its absolute limits can reach up to Ψ12 cruise, on models specifically upgraded. This has proven to be an immense logistical burden for the Federal fleet, as the Thor class is now significantly slower than most other Terran ships. 
The Federation is currently working on a replacement for the Thor class. Just as the Hades-class destroyer is meant to replace the Raijin and Agamemnon destroyers, the Horus-class frigate is meant to replace most Terran frigate classes. The Horus will be larger and much more technologically advanced than the Thor class. It will integrate multiple energy based weapons, which the Thor class lacks altogether. It will also be better equipped with point defense systems, so as to be able to function independently without a dedicated point-defense escort - one of the main downsides of the Thor class. It will also be made so as to function with the new generation of Terran ships, such as the Loki and Hades classes. 
With over 20000 units built, the Thor class is by far the most produced warship in history. For comparison, the second most produced ship, the Ur-Han Jedduk corvette, has had 12000 units built. The Thor class, whilst primarily utilised by the Federal Navy, has also been used by several Terran allies. These most notably include the Tesrat Republic and several Dominions. The number of countries using the Thor class is however only going to grow. As it is slowly being phased out in favor of the Horus, Thor class ships are being refitted with the most modern equipment available and being sold. Some of these units are even sold to nations at odds with the Federation, such as the Ur-Han and Mirdesh Federation.