GTVA Colossus ship type debate
Ambiguous evidence in canon
In-game dialog simply calls the Colossus a warship. Warship is used as a generic term in FreeSpace to designate all combat-capable ships cruiser size and above. In the second stage of The Great Hunt's mission briefing, even a Rakshasa is simply called a "warship."
TVA battle ship of massive proportions
A block of text in the Colossus cutscene reads: "TVA battle ship of massive proportions". There are a number of inconsistencies between confirmed canon information and the FreeSpace 2 cutscenes. The Colossus cutscene itself is not in accordance with, for example, the size and armament of the in-game Colossus. The introductory cutscene of FreeSpace 2 also portrayed the SD Lucifer with its beam cannons misplaced. An out-of-universe explanation for this is that the cutscenes were made in a separated studio, and was not overseen by Volition.
In a forum thread at Hard Light Productions, fans have suggested the juggernaut designation. 59 votes were cast on juggernaut, with superdestroyer being second with 17 votes. The community seems to agree that the juggernaut designation would be fitting for the Colossus, because it is the Terran equivalent of the Shivan Juggernaut Sathanas. The Sathanas class is explicitly called a juggernaut, so logic would dictate that a warship that is on par with it in effectiveness should also be classified as a juggernaut.
Please note that, although juggernaut may seem the best option, it's not to be used extensively. It would also be advisable that ships or any other assets with comparable capabilities and dimensions don't share the same designation. A good example is the German U-Boat used in World War II, although they were the direct counterpart of Allied submarines, different names were indeedly used to emphasize the differences between the two sides. Juggernaut is not a conventional term. It comes, in fact, from Sanskrit] and was one of the names of Krishna, so the chances that its usage would not fit with the GTVA Colossus are to be taken into serious consideration, despite the common tendency to call the Colossus a juggernaut.
Superdestroyer is used for the SD Lucifer to distinguish it from other destroyers in FreeSpace 1. The Colossus may be distinguished from other destroyers in FreeSpace 2 in such a way. However, designating the Colossus a superdestroyer would imply that it is on par with the SD Lucifer. This comparison is difficult to make, however, because the Lucifer does not appear in the FreeSpace 2 campaign. Its primary armament in the table files consists of two SRed beams, but given how unfavorably these compare to the Lucifer's armament in FreeSpace 1, it seems likely that the SReds are a place-holder or were chosen early in development and never revised.
Super capital ship
The Colossus is given the -supercap flag in its ships.tbl entry. It is the same flag that is used for the Sathanas, which is explicitly referred to as a juggernaut in game, and the Lucifer that is designated as a superdestroyer. Additionally, this is a generic designation, often used to describe any combat vessel larger than a destroyer. Nevertheless, the Colossus is never explicitly called a "super capital ship" in any text in FreeSpace 2.
Considering the amount of effort and time it requires to build one of these ships, it is fair to say that there will never be another Colossus. The next ship of it's magnitude would have different/improved designs, technology and looks. Remember that the ship was designed to be the ship to end all wars. So assigning it a class would be superfluous - there really isn't a need to have two of them, let alone enough of them for people to need to be told what it's class is to be able to identify them. So simply name it the collossus, and be done with it. (And if you really want to have more then one of them, give each of them a unique name.)