PSA: Faction Reputation (minor spoilers to prevent grief)

[Null Winter]

Short summary:

If you're wondering who to build Reputation with via trading and planetary construction to get those unique goods, then try to build up with the Fellowship Of The Void and the Drusians since the other factions have more lenient ways to gain rep.

I believe 164 Profit Factor is the highest PF threshold you need to clear to demand the surrender of unique items once your Reputation tier is high enough with everyone, but most factions cap at asking for around 125 for their best stuff.

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Longer summary:

The Navy is eventually maxed out effectively for free, since they don't accept the same goods anyone else does and instead only take trophies from space battles; there's no conflict of goods and no quest decisions will have the Navy and another faction's rep competing. A couple of colony constructions also grant massive Navy boosts along with other permanent perks, including given you a permanent expendable Navy ship ally in all space combats who will respawn after soaking fire for you. I'm honestly not sure why they're even a faction, since you rarely interact with them directly and only ever encounter generic faceless NPCs as part of minor events; it feels like dropped content or shoehorning in something required by GW for the license.

Kasballica will basically be maxed out almost for free: multiple major story events and companion quests forcibly involve them and grant opportunities to earn rep, planetary construction options grant a lot of rep and usually don't compete with other options, and they love that insane amount of trash Dark Eldar gear you'll have by the end of the third chapter. Many of their options for rep gain will feel scummy if you're playing Dogmatic, so if you're diehard roleplaying then you're going to minimize your interactions with them anyways.

The Explorators are easy enough to please since they accept a wide variety of stuff that doesn't overlap too much with other factions' preferred goods, and a couple events and construction options increase their rep. They also will offer Profit Factor buffs as a separate trade-in option if you manually talk to their in-person representative to hand over your planetary exploration charts.

Fellowship is rough if you don't know what's coming up; their top 3 preferred goods are the same as Kasballica, so it's ultimately better to give those goods to the Fellowship to make up for how much rep you can get with Kas just by playing the story and building constructions. Several of the Fellowship's options compete with other options and will also feel extremely out of place if you're doing a pure Dogmatic or even an Iconoclast playthrough.

The Drusians suck and are impossible to please. They have only a single rare trade good that they value at 500 rep, few planetary construction/event options please them, and those options seem to always conflict with something else.