The following is a list of Final Fantasy XIV superbosses. The vast majority of bosses that can be considered superbosses come from Extreme-difficuty 
Trials and Savage-difficulty 
Raids. Bosses encountered in these duties represent the hardest content in the game at the time of their release, with Savage raids generally seen as being more difficult than Extreme trials. In addition to these, the Binding Coil of Bahamut, Second Coil of Bahamut, and Final Coil of Bahamut raids, which do not have Savage modes with the exception of the Second Coil, could be considered superbosses as well, as they were very difficult and represented the hardest content of A Realm Reborn before the Savage raid difficulty was fully established in Heavensward.
As players improve in levels and equipment, duties routinely see drastic falls in difficulty, particularly given the Duty Finder option to run these duties "unsynced", allowing players to run these duties without enforcing level or item level syncs. Duties that once represented the pinnacle of difficulty for eight players can often be defeated by a single player under these circumstances. Players seeking to challenge these duties at their original difficulty can instead use the Duty Finder option to enforce a duty's minimum item level, syncing all participants item level to the minimum required to enter the duty.
Stormblood introduced 
Ultimate raids, high-end raids that host a gauntlet of bosses from previous duties that must be defeated in a single attempt. Failing will reset the entire gauntlet. Ultimate raids can only be unlocked by completing the prior Savage raid tier, and unlike other duties cannot be run unsynced. Ultimate raids are designed with the purpose of being the hardest content in game.
In addition to trials and raids, certain open dungeons, high-end FATEs and Hunts can also be considered superbosses by virtue of the sheer amount of players needed to complete them. Open dungeons require parties of anywhere between 48 and 72 players depending on the dungeon, while FATEs and Hunts have no limit on how many players can participate as they occur in the open world.
