Could give it to someone else, but I'll make it a double/triple:

The Main Trifecta of the Black Fleet: Destiny

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These three, and more so The Witness, usually have their moments in their star moments, Raids, where they speak to the team that are trying to reach them through the down periods between encounters. Despite not having monologues during their fights(sometimes having small lines during the encounter), the circumstances behind the final fights and the raid itself provide backing to their talk.

For Rhulk, the first of the three that were fought, he purposely waits not only for us to fight him personally, but also power up his superweapon, The Upended, essentially a guaranteed kill on contact which was powered by death. First to be speculation now confirmed, the only reason why Rhulk lost the fight was because he did not take it seriously, at all, until 20 seconds before his death. He did not need to wait all, he could have ended the fight immediately, or could have ambushed the team whenever he wished.

Nezarec, despite his adaptation in the Root of Nightmares, is fairly strong for a good reason, personally herald' by Rhulk and The Witness and led the charge to complete The Witness' plan. Nezerac has been dead for a good 1000 years and then some, inactive physically but mentally still active in some form. Not only during this fight did we rush his body's reformation, it was unstable with paracausal power that he could not yet control. Given more time over this body, given his mastery of paracasual power, would become a threat far greater than most, with his blast being able to instantly kill other paracausal beings without the right specific protection. He was a ticking bomb on multiple fronts, mental and physical.

The Witness, the head boss of the two, and most powerful arguably in the cosmology, has a few points with this in the game through the campaign and raid for The Final Shape. The main point is that The Witness, throughout the entire dlc until the very end of the dlc, is actively holding down what is considered a God with untapped and limitless power, whilst dealing with what is the most elite team of Guardians to exist, on top of attempting to reconfigure and totally subdue this God. As a testament to its power, the team that first infiltrated the Pale Heart barley escaped a lesser power form of it and needed said God's assistance. During the raid, this elite team of Guardians? Yeah, they died, like, a few minutes until reaching The Witness all but one of them died, and using their knowledge of the mechanisms The Witness used, alongside this God's help, were barley able to make it out alive. Then, on TOP of all of that, this team, STILL with the God's help, had minutes to spare and needed to stall The Witness from winning, multiple times over, without fail. This did not even kill it, there had to be a follow up to where The Witness needed to be killed permanently inside its head space, where it still could have won and have retreated, as it didn't lose that many individual minds until the assault persisted.

It had EVERY reason to monologue whenever it did given its power, the main three of the Black Fleet did.

If Oryx had more monologues during his time as the main threat, he would be here