If we're going off of damage output, probably the unknown sniper ship that was present at Reach. If we're going on damage it can take, then the Super Destroyer.

Well, technically, all covenant ships are reverse engineered Forerunner weapons/starships.

Modern UNSC-Covenant naval engagements aren't really my speciality in Halo lore, but I'll do my best to logically approach this. In terms of size, Infinity is just under 19,000 feet long. A CSO-class Supercarrier is [i]95,000[/i] feet long, so the Infinity is practically dwarfed by that particular behemoth. Of course, it's not all about size. Infinity's MAC packs a punch, it managed to even make a hole in the Mantle's Approach - a small hole that was fixed by the vessel in under a minute, but it's more than what Lord Hood's fleet of MAC attacks on the Keyship managed in Halo 3. So Infinity's firepower is pretty solid as it has 4 MACs, Archer missiles, Rapiers, Howlers, AA guns, HAVOK nukes, and so on. But, at the same time, the 'O' in CSO stands for 'Ordained' which means that the ship has seen some augmentation with Forerunner technology - apparently, this is to do with the Covenant's weapon systems being upgraded with Forerunner tech. Bearing in mind that the Infinity has taken 3 direct hits from an energy projector (a weapon that is used to glass [i]planets[/i]) and was still operational, I daresay that Infinity would be able to hold its own. Covenant pulse lasers deals damage in the kilowatt range, and we don't really know exactly how strong Infinity's shields are but I doubt they'd hold up long after several barrages from those. In all, I'd say that the Supercarrier would probably come out on top. If the Infinity managed to get the Supercarrier's shields down and pelt it with Archer missiles, then they might stand a chance, but the best means to take out a Supercarrier from the UNSC's point of view is to take it out from the [i]inside[/i].