The Viscount-class Star Defender is Super-capital Ship that is available to the New Republic in Era 5 alone. It has an impressive armament of 540/54 Heavy Turbolasers, 540/54 Turbolasers, 200/20 Ion cannons, and 100/10 Concussion Missile Launchers.
Even though she was destroyed (by an A-wing), it took the entire Rebel Alliance Fleet at Endor to only just disable the Executor's shields which led to the idea of building a similarly sized ship to counter the terrifying Star Dreadnoughts almost immediately after the Battle of Endor. By the time the New Republic had formed and established itself, Admiral Gial Ackbar brought up the fact that the Star Dreadnoughts were the most dangerous threat to New Republic Navy. Proposing to build a counter ship, it would take many years for it to develop and Thrawn's Campaign against the New Republic, followed by Operation: Shadow Hand a few months later, certainly didn't help.
It was only when the rogue Star Dreadnought Intimidator, under the control of the genocidal Yevetha, presented a direct threat to New Republic stabilization that the program was rapidly pushed forward. It would prove to be a monsterous task. Nothing of this scale was ever done before by the New Republic, but Mon Calamari Shipyards finally began construction of Viscount-class not long after the Black Fleet Crisis.
The first ship of its class, the Viscount entered service around 25 ABY, just before the Yuuzhan Vong War began. The physical appearance of the ship was that, while its length was slightly smaller than an Executor's, Star Defenders had more mass to them. Because of advanced Mon Calamari ship building techniques, the shield generators were kept inside its hull and allowed the Viscount to utilize a smaller crew then its would-be rival. The Viscount itself would prove instrumental in the Yuuzhan Vong's attack on the Galactic Alliance Capital, Dac.
The Viscount-class Star Defender is the most powerful ship that can be used by the New Republic, and likely one of the most powerful ships in the game. It is essentially an SSD and should be used in a similar fashion. But in a one-on-one confrontation with an Executor-class, the Viscount will come out on top as it has superior Mon Calamari shielding.
However, even the Viscount has a weakness, which is shared with its bitter rival. It has, almost, no defence from the rear! Other than that, and using your own SSDs to fight against it, it'll be very hard to defeat a Viscount should you cross paths with one. And, unfortunately for New Republic players, there is a lifetime limit of three of these ships and only one can be active at one time, meaning that, if the risk of losing a Viscount is too big, call for a retreat as quickly as possible (and hope your decision wasn't too late). But in short, the Viscount-class Star Defender is the best cure for those Executor Star Dreadnoughts in Era 5 and should be built the moment it's available.