Who to Give spare DC

Who is your most used unit in your opinion in that group? and give DC to that unit.

otherwise Give it to Effie, Since she has armored privileges(armor skills and buffs) she has the "best" potential in that group.

Effie is the best candidate. Also is great for Arena points once you +10 her.

I'd say Soleil is the best for it, for color balance.
While -res migth not be ideal, she should be able to take a hit with a refine weapon and her counters will be amazing.

However, you could concider some other units, going for balance a red would be my pick.
Fir with moonbow or iceberg can take hits well and fire off strong specials with her nameless blade
Maybe not the 100% best choice, but Zephiel isn't too bad of an option. He's got the best magic bulk out of the red sword armors with his refined Eckesacks (58 hp+24 res+ 6 res from DC). And of course his physical bulk is just amazing. No archer can do anything with him

If you can wait, put it on Swordhardt. +Atk with DC/Vantage and horse buffs allows him to eliminate any challenge in PvE. Mines +Atk at +3 and he could probably solo the entire game if I wasn’t concerned about HM.