The Mon Calamari, an ally of the Alliance, in my opinion, had produced the best capital ship for the Rebel Alliance in their quest against the Galactic Empire.
The Rebel Alliance was at best lacking the money and technological expertise the Empire had enjoyed. With Kuat Drive Yard and Sienar Fleet System backing the Imperial, the Alliance faced a haunting prospect of losing to a better-equipped enemy. Therefore, they were willing to accept anything from anyone as long as their fight against the Empire did not diminish.
Their luck changed when a sympathizer, Incom, the manufacturer of Z-95 Headhunter, turned in X-Wing blue print to the Rebel Alliance. While the Alliance found a new all-round starfighter, they at the same thing found a new friend – the Mol Calamari. The Mol Calamari, especially Admiral Ackbar proved to be a great asset to the Alliance. Through the Mon Cal, the Rebel was introduced to a new class of starship – Mol Cal (MC80-a) Star Cruiser.
Mon Cal was the main starship class for the Rebel Alliance. It played an important role during the Battle of Endor. A few of them were destroyed by the Second Death Star powerful beam but most of it survived to see the final defeat of the Empire at Coruscant.
The ship has the most peculiar design. By far, it is the only capital ship in Star Wars Universe that would pass an aerodynamic test. Also, not one ship in the Mon Cal class is similar to one another. Like every being, every different ship in its class is unique.
The Mon Calamari is a fan of redundancy. The ship has several shield generators and thus, having a high shield refresh rate compared to the Kuat Drive Yard’s infamous Star Destroyer. Nevertheless, a typical Star Destroyer has a stronger shield. Apart from depending particle and ray shield, the Mon Cal armor has the most impenetrable hull.
Stressing once again on their belief in redundancy, massive armor plates were install on each ship; every armor is reinforced with extra plating. Furthermore, the Mon Cal has a huge drive array, making it one of the fastest ship in its class.
Later during the Battle of Endor, Admiral Ackbar took command of a Liberty class Mon Cal, a larger derivation of MC80-a. This ship was so fearsome that even a Star Destroyer wouldn’t want engage it hastily.
Image taken from http://mitglied.lycos.de/STARWARS_Blueprints, et la The Essential Guide to Vehicles and Vessels (Bill Smith and Doug Chiang).