The UNSC Supercarrier is a heavy warship classification within the United Nations Space Command fleet. There were only a few constructed, the only one mentioned is the UNSC Trafalgar, which fell at the Battle of Reach. This warship classification was likely to be on the same level as the UNSC Marathon-class Cruiser-Carrier or higher in terms of overall power.
Speculation
Although not much at all is known about the Supercarrier, it is reasonable to think that this ship would of been one of the most powerful ships, if not the most powerful ships made by the UNSC known to date because it is believed that the UNSC Carrier is more powerful than the Marathon-class Cruiser-Carrier based on information from Halo: Fall of Reach, Halo: First Strike, and Halo: Ghosts of Onyx, and its designation as a supercarrier most definately makes it more powerful than the standard UNSC Carrier.