Champion with Blade Ally
... is the only class that meets all your criteria. In particular, a Champion with a Good Cause and Blade Ally doesn't just meet, but actually stands out in almost every criteria.
Qualifies by level 2 (all martial weapons)
This criteria doesn't narrow it down at all. Barbarian, Warrior Muse Bard, Champion, Fighter, Gunslinger, Inventor, Investigator, Magus, and Ranger meet the martial weapon prerequisite out of the box. The Wizard needs to take1 the Weapon Proficiency feat twice to meed the prerequisites, while everyone else only needs to take1 it once.
Ability Score Constraints
Can have a decent Charisma score (starting with at least 14, so not very MAD) [...] At least 11 HP per level after Con [...] At least AC 18 on level 1 without shield [...] Has decent Strikes, in case they are grabbed or the enemy has AoO
These criteria all rely on ability scores and are affected only by the classes key ability and armor proficiency. Although the cutoff points for these criteria are somewhat arbitrary, we can assess them objectively.
- At least 14 Charisma.
- Decent Strikes require strength or dexterity, let's say at least 14 in one of them, similar to decent charisma.
- I will consider at least 18 in the class' key ability as an implicit criteria, because a class that can't achieve this is hardly the best for Marshal.
- At least 18 AC at level one can also be framed in terms of the class' armor proficiency and, to avoid the armor penalties, some minimum combination of strength & dexterity: 8 & 20 for unarmored (or 8 & 16 for Monks because they are expert), 10 & 18 or 12 & 16 for for light armor; 14 & 14 or 16 & 12 for medium armor; 16 & 10 for heavy armor.
Unarmored's 8 & 20 is impossible before level 5, but Sorcerer, Summoner, and Witch can take1 Armor Proficiency to wear light armor, while Wizards can't because they already need to take1 Weapon Proficiency twice.
- At least 11 HP per level can also be framed in terms of class and constitution: Barbarians need at least 8 constitution; Champions, Fighters, Monks, Rangers, Summoners, and Swashbucklers need at least 12 constitution; Alchemists, Bards, Clerics, Druids, Gunslingers, Inventors, Investigators, Magi, Oracles, and Rogues need at least 16 constitution; Sorcerers, Witches, and Wizards fail outright because they would need at least 20 constitution, and not even the Toughness feat can salvage this.
Barbarian, Champion, Fighter, Monk, Ranger, and Swashbuckler easily meet these criteria. Bard, Gunslinger, Magus, Oracle, Rogue, and Summoner can also meet all these criteria, if just barely by choosing appropriate ancestries and backgrounds.
On the other hand Sorcerer, Witch, and Wizard failed outright as mentioned previously. Alchemist, Cleric, Druid, Inventors, and Investigators cannot meet these criteria, even if they take1 the Toughness feat and choose appropriate ancestries and backgrounds the best array they can get is 18 14 14 14 10 8, and it's impossible to assign those scores to reach 18 in AC, 18 in key ability, and 14s in strength/dexterity, constitution, and charisma.
Down to 12 contenders: Barbarian, Bard, Champion, Fighter, Gunslinger, Magus, Monk, Oracle, Ranger, Rogue, Summoner, and Swashbuckler. Going forward I'll disregard the other classes.
Disregarding Class Feats
Not feat starved
This one is tough to assess, because you'll only be feat starved if you desire more feats than you can acquire, but the desirability of feats is partially subjective and often depends on the specifics that go beyond just class and archetype.
However, this criteria implies that you wish to take Marshal feats at almost every opportunity, so a good approximation for this criteria is to simply disregard class feats when considering the other criteria. Hence going forward I will focus on class features.
Want [...] to be in the frontline because your aura is usually 10 feet
No surprises here. Any class that wants to consistently make melee Strikes or short-ish ranged Strikes wants to be on the frontline, or at least right behind the frontline from where the aura can still reach allies.
Bard, Oracle, and Summoner don't meet this criteria: it's certainly possible to limit your repertoire to short ranged spells, but there's really no incentive to do so; most Eidolons are meant to be in the frontline, but the Dread and Inspiring Marshal Stances originate from the Summoner, not the Eidolon.
The standout for this criteria is the Champion with a Good Cause, because the 'range' of the granted reaction roughly matches that of the Marshal's aura and that will further incentivize cohesive positioning from the whole frontline.
Not action starved, does not have anything better to do with their actions
Marshal has several go-to actions that can be used every round to great effect like Cadence Call, Rallying Charge, and To Battle! Every class also has it's own go-to actions, whether they are basic actions enhanced by class features, or new actions granted by class features. If we want to use Marshal actions often we need determine what the go-to actions for each class are and how much they compete for action economy.
- Cast a Spell is the go-to action for almost every caster. The number of actions is typically two, but can also be one or three. Cast a Spell is particularly competitive when used with a focus point or one of your few highest slots, though some spells remain very competitive regardless of what slot they use, and Cantrips are also competitive. This doesn't leave much room for Marshal actions, especially the two-action ones. Bard, and Oracle fit snugly in this category, and Bard in particular has the very spammable Inspire Courage.
- Strike is the go-to action for almost every martial, with the MAPless Strike being especially competitive. The second Strike is competitive, but not nearly as much as the MAPless Strike, and the Strikes at full MAP generally aren't competitive at all. That means that there's one or two actions available for Marshal actions. Moreover, Rallying and Coordinated Charges subordinate the MAPless Strike instead of competing with it!
- Some Rangers enhance Strikes unevenly MAP-wise: the Flurry Ranger specifically enhances Strikes with MAP, leaving less room for Marshal actions; the Precision Ranger enhances only the first hit, but if the first Strike misses, the second Strike will be more competitive than normal again leaving less room for Marshal actions.
- Some martial classes subordinate Strike into a different go-to action. Unlike normal Strikes, these actions are incompatible with Rallying and Coordinated Charges.
- For Magus, it's Spellstrike, which also subordinates Cast a Spell. In addition being two-actions, it also must be reloaded with another action, leaving very little room for Marshal actions.
- For Monk, it's Flurry of Blows, which is also incompatible with Cadence Call and To Battle! due to the Flourish trait.
- For Swashbuckler, it's Confident Finisher.
- The Summoner's go-to actions are both Strike and Cast a Spell. Even with Act Together the there will be less room for Marshal actions, especially the two-action ones.
- Some classes have other go-to actions besides Strike or Cast a Spell, which further compete against Marshal actions.
- For Champion with Shield or Steed Ally, it's Raise a Shield or Command an Animal respectively. While riding, Command an Animal can be used to effectively Stride twice, but unfortunately this benefit is incompatible with Rallying and Coordinated Charges.
- For Gunslinger, it's Interact to reload.
- For Outwit Ranger, it's the Edge's listed Skill Actions.
- For Rogue, it's any action that makes a foe flat-footed, though some Marshal actions can be used for this purpose, instead of competing.
- For Swashbuckler, it's any action that generates Panache.
The Dread and Inspiring Marshal Stances compete strongly against other actions that are best used in the first round and a few classes have such actions.
- For Maestro Bard, it's Lingering Composition.
- For Barbarian, it's Rage. Fortunately, no Marshal feats have the Concentrate trait.
- For Magus, it's Arcane Cascade, which is also a Stance and therefore completely incompatible.
- For Ranger, it's Hunt Prey, which ideally can be used during exploration instead of the first round.
The Marshal's Attack of Opportunity, Topple Foe, and Target of Opportunity compete with other reactions, but having additional triggers at your disposal also increases the odds that you'll spend your reaction at all. Hence, acquiring more reactions with differing triggers is still positive for your action economy.
- The Bard has Counter Performance;
- The Champion has the Champion's Reaction associated with a Cause
- The Champion and Fighter both have Shield Block, though the prerequisite Raise Shield does compete with other Marshal actions.
- The Fighter has Attack of Opportunity, which isn't a different trigger per se, but makes Marshal's Attack of Opportunity unnecessary.
- Some of the Oracle's Focus spells, depending on Mystery and Domain choices.
Speed is also a relevant to action economy because with greater speed you'll have to use fewer Move actions in the long run. Monk is easily the best in this regard, and only Swashbuckler with Panache can match the Monk.
Champion with Blade Ally, and Fighter stand out in this criteria, because their only go-to action is Strike and they have reactions to boot. Rogue is second, because some Marshal actions to flank in place of a different go-to action. Barbarian is a close third, because it doesn't have any go-to actions besides Strike, but is held back by the need to Rage.
The other classes have go-to actions that either eat into the action economy more than necessary, or directly conflict with Marshal actions.
As a side note, the choice of weapon can also be significant for action economy. Weapons with the Fatal or Deadly traits make Strikes at MAP comparatively less appealing. Ranged weapons and returning thrown weapons can reduce the need to Stride. Weapons with a reload entry other than 0 obviously have terrible action economy, though feats such as Running Reload can alleviate it.
Preferably has a use for that Charisma besides being a Marshal
- Bard: Key Ability, Spellcasting, and reliance on Performance
- Oracle and Summoner: Key Ability, and Spellcasting.
- Champion: Spellcasting (only Lay on Hands or Touch of Corruption) and persistent alignment damage (granted by their Cause at level 9).
- Pistolero Gunslinger: reliance on Deception or Intimidation
- Outwit Ranger: reliance on Deception and Intimidation, among other skills
- Eldritch Trickster and Scoundrel Rogues: Key Ability, Spellcasting (only two Cantrips) for the former or reliance on Deception for the latter
- Battledancer, Braggart, Fencer, and Wit Swashbucklers (reliance on Performance, Intimidation, Deception, or Diplomacy respectively).
Bard, and Oracle stand out in this criteria, because most of what they do uses charisma. Outwit, Braggart, and Wit would stand out more if their bonuses could benefit the activation of Dread and Inspiring Marshal Stances.
Choosing charisma for the Rogue's key ability is an interesting option, but I wouldn't recommend it considering that a Marshal Rogue will generally use dexterity far more than charisma.
The last two criteria largely disagree on what class is best for Marshal and in retrospect it's obvious why: a class that "does not have anything better to do with their actions" also doesn't have any worthwhile actions that use charisma. Still, that leaves passive bonuses that use charisma, which the Champion receives at level 9. Moreover, the Champion with a Good Cause and Blade Ally tops all the other criteria too: it meets the dedication prerequisites and the ability score constraints without needing General feats, its reaction gives allies an additional reason to stay by your side, and most of its actions can be spent on Marshal actions.
Fighter is worth mentioning, despite not meeting all criteria, because it will activate the Rallying and Coordinated Charges' on-hit effects more often than other classes.
Rogue only barely passes some of the criteria, but I've thoroughly sold myself on the idea of repositioning an ally into flaking with To Battle! in order to land a Sneak Attack (though depending on the size of the enemy, the aura might not reach the flanking ally).
- Humans can take any two General feats via their General Training feat and their Versatile Heritage. A couple other ancestries can choose any one General feat by nabbing the Human's General Training feat.