Who is worth leveling up?

Artoria 100%, Orion for special situations, Chloe for waifu reasons

Chloe has instant NP charge (+50% NP bar with cooldown on 6 turns, at level 10).

This, plus Projection, makes her almost as spamstatic Arts Archer as Archoria (Kaleido Sapphire MLB or Azure Magical Girl make it even sweeter...).

Both are decent, end leveling Archuria if you have the mats needed. Then Chloe, then end Orion.

Also, do level David when you are free to do it, his Harp of Healing is a hell of a survivability skill. And his ST NP is solid.

Play long enough and everyone can be leveled up. Exp isn't that hard to get. Then play with them all and skill level up your favorites. David has been a go to for me forever since I love his skills, group heal and 1 turn evade saved the day many times. Maybe that will change now that I have Chloe and beach pirates for st np, but I think I'll still use him

I wouldn’t write off Orion or push him into a male killer only role. Unlike Euryale, his NP does the same damage to all servants and doesn’t get a boost to just males. His male killer role comes from his second skill only, so he can still do a lot of damage to any enemy type since he has SSR stats. His base attacks will hit the hardest and he fits in well in a high damage Arts role when you boost his NP with Tamamo. Squirtoria would be my first, then Orion and close out with Chloe.

Squirtoria. She is one of those s tier servants.

Chloe's NP will probably hit harder, but Squirtoria can probably spam hers more often (which is basically the point of using an arts archer anyway).

For the priority go for Squirtoria first, later Chloe will pick up the pace. These 2 archer is already broken and try not just bring one main Archer in your team. I mean when things go wrong and Squirtoria died, Chloe can come to substitute her or Emiya and the other were good.
Some hint :
1. Robin hood is a Nuclear weapon.
2. Billy the Crit.
3. David is a monster.
4. Arash Stella is a meme for some reason.

Experienced FGO often make a joke when using the low star archer, because they're broken and thrust me their ability make them a legend.

squitoria, back to back NPs is fun :D

IMO Artoria, the NP refund is amazing.

Go for Altria 1st, Chloe 2nd. If you need male killer that loli goddess do better job than Orion. Her NP is more spammable than Orion after strengthening in art team