Who to give bold fighter.

Halloween Jakob since he's speed is so slow. I think you listed the best to worst candidate by order unconsciously already. Henry has 37 speed with boon so he's the least needed candidate.

honestly I would wait until you get a good Effie but you can also give it to Jakob if you really want. but in all honesty I think your Jakob would be better off with a close counter vantage/wary fighter build if you can afford it

H!Jakob can be monstrous with Brave Bow+Bold Fighter, basically Bold fighter alone illuminates the need for Death blow and you could just run Close Counter if you want to, because he'll be able to proc Bonfire, Ignis or Aether in a single round of combat.

Hector is another good choice though, because that means your opponent can never safely bait him, because on the opponents point of view, player phase Hector has quick riposte while enemy phase Hector has bold fighter to double.

Honestly I still think the Black Knight is the best choice for this skill in particular.

Sure he has enough speed to double many other units, but with Steady Breath in the A slot and Bold Fighter in the B slot you simply cannot avoid being hit by Black Luna without a blue mage who can OHKO him. This alone gets you many arena defense wins.

That being said though, I think you should put it on Hector from that list. It's solid on everyone, but on Hector not only is he dangerous to attack thanks to Armads but you also can no longer let him run into you to avoid damage.

Only thing to consider is that Hector is still ridiculously powerful with Vantage, so if one of the others is lacking in some regard maybe think about boosting them with Bold Fighter, getting them to the same level as the others that is.