I currently have Emiya, David, and Euryale max lvl. Trying to decide who I'm going to raise next. Currently still need leveling are
Arash NP5
Billy NP4
Ko Gil NP3
Robin NP1
Billy, Robin, Ko Gil, Arash would probably be the best order for you. Billy isn't as strong as Robin, but he still hits hard, and with his NP level being better, it will probably serve you better in the long run. Ko Gil is a middling archer but he's a better all rounder than Arash. Arash is good though, so don't ignore him for being st the bottom of the list.
I will say it depends on what you focus more on. If you don't have many trained up AOE Berserker, do Arash first. If you want to focus on higher difficulty quest, Robin is important. Billy might be better now since you have him with higher NP level, but once you got Robin's NP level up, his extra damage to poison debuff is insane.
Kid Gil's NP is AOE and since you got Emiya already, it is irrelevant.
Also, if you can wait, then wait for Chloe because she is one of the best service ones out there.
You have all arts archers. This means robin, despite his great np, is low prio as he also does no dmg outside of np. Billy should be highest prio with his great st dmg. Next is either arash or ko gil. Both are great.
