Which agent and/or faction do you think has been the most useful asset to Phaethon so far?
Rn my money is on either the Cunning Hares or Victoria. Specifically Nicole and Lycaon.
Nicole has been a ride or die since the beginning and has offered a ton of useful intel to Phaethon throughout the story. She also kickstarted the whole plot and has informed Phaethon about some of the event quests that have occurred in various patches. The rest of her team has also been equally loyal and reliable towards Phaethon as well.
Meanwhile Lycaon is also up there for similar reasons. Very loyal towards Phaethon, and has given them a lot of useful intel. His intel is arguably even more useful than the Cunning Hares since his group works directly for the Mayor and is privy to a lot more classified info. His connections with the Mayor also allowed Phaethon to upgrade their ether aptitude and get them in contact with Yunkui Summit to help further train their abilities.
Some other top contenders I think are Pubsec, Section 6, and Yunkui Summit. Either because of their status, connections, or cause they have a Void Hunter amongst their ranks.