In my very limited experience, I would say either D'Artagnan, so you can unlock sniper shot and get a better farming soul overall, or Mordred, if Black propaganda is required.
Since D'Artagnan is directly unlockable in your case (you need 180 more sp for Mordred), I would recommand the former over the latter, but I may be wrong.
Edit: I will also post my question about the "which soul should I use". New player, with Belial so currently Fire focused, with Ryu-oh on the side. First, I unlocked Hercules because everyone said it was a powerhouse and she really is. Then, I unlocked D'Artagnan for Sniper shot (on Hercules) and maybe try to get the bonus drop when farming. Currently, my fire team is Agni/Hephaestus/Raguel/Brynhildr.
Basically, debuff wise, I'm at ~35% def debuff and 35% attack debuff.
I will also be more or less able to buy a soul weapon soon, and that's when I started thinking. If instead of Hercules with the axe, I run D'artagnan with the HP gun and ryu oh, I can immediately cap both the def and attack debuff (meaning I will take 23% less damage overall from bosses and do 30% more damage), I get the black propaganda effect, and I can use more or less anyone (since the major debuffs are covered at max effect) instead of currently mandatory Hephaestus + Raguel (Raguel has a good punch against raging enemy, but Hepha is quite meh out of her debuff, since Brunhildr has already a A-frame attack buff). I mean, I guess it's not really useful currently since I have no one to use instead, but maybe in the futur?
Or I should just get this hercules axe, don't ever use Ryu-oh on a fire team, get the def debuff on burst (which apply after her own burst I guess?) and switch between 40% (Axe + Hepha), 45% (Axe + Sniper shot) or a way overcapped 60% (Axe + hepha + sniper shot), and call it a day? The main problem I have with that is how the skill effect from the axe is quite "meh" for me right now (Hercules is always way ahead of other burst-gauge wise) while both effects from d'Artagnan gun are useful (the def/attack debuff and the DATA buff).