Maxperson
Morkus from Orkus
There is nothing suboptimal about the fighter and my goals. Were combat important to me, then and only then would you be correct.because that's a cavalier.
but seriously, ignoring that you're already pigeonholing your options down to suboptimal ones by single classing fighter, you have a bunch of better options just in the fighter. cavaliers, again, are much closer to archetypical knights with how they work with a mount (knights were cavalry units), though cavaliers have their problems what with mounts not always being practical. the samurai has a similar persuasion boost to the banneret, but lets you really lean into the fighter's main advantage (attacking). and the battle master has options for (temp) healing, protecting your allies, giving them additional attacks, and gaining bonuses to various skill checks (including persuasion), without having them take your base class resources like the banneret does (and sure, the skill checks do take subclass resources where the banneret's persuasion boost doesn't...but let's be honest, you're probably not your party's main face as a fighter regardless of subclass).
in comparison, the banneret basically boils down to "when you use your main class resources, you can give your allies a discount version of the same effects". and that's not terrible, but it means if you want to get good use of your subclass, you need to time what would otherwise be pure self-buffs with whenever it'd also best benefit your party, which makes your base class features significantly more situational then they otherwise would be. no other fighter subclass does that. the only part of the banneret that doesn't make you worry about that is the persuasion boost, which...i mean, okay, cool, i can be an off-face, but i could also do that as a samurai or battle master, and if i do that then i don't need to worry about saving my base class resources for when i can buff my class.
so in short, you can get a lot of similar effects from other subclasses, those subclasses can represent the concept of knighthood at least as well if not better, and those subclasses don't force you to completely change how you use your base class abilities. so, hey, pick banneret if you want, but you can definitely do better.
and i didn't even get into echo or eldritch knights...
As for the Cavalier, it doesn't work since everything it gets is combat related and I don't care about combat. Knights had other skills which the Cavalier doesn't have, but the Purple Dragon Knight at least has some of. The Samurai and Battlemaster are not knights, though the Samurai does have abilities that help outside of combat. I want a knight, not a Samurai.
Also, if people are playing a game that has any sense of realism at all, there is no party face. A king would be insulted if every time he asked the Barbarian something, the Bard answered, and NPCs don't just talk to the one person you want them to. My knight will talk to people and try to persuade them of things, including important people. I'm not going to allow someone else to do all my talking for me.
Names mean something. A Samurai or Battlemaster will never be a knight subclass, because they are not knight subclasses. They are a Samurai and a Battlemaster. A Cavalier could be a knight, but I'm not interested in only combat, so none of those can match my concept. I'm also glad you didn't get into Echo or Eldritch knights as they also don't match my concept and are in fact worse at it than the Samurai. I don't want magic involved.
You still haven't proven or even attempted to prove my choice to be objectively bad by the way. Simply listing out things that might possibly maybe fit is not any sort of proof.