Who to grail?

I mean, if you're grailing for efficiency, you're playing the game wrong, but whatever. Here are my suggestions:
OG Cu lancer, without a doubt. He's just as good, maybe even better in some cases, as herc in the backline. He will never die. I'm grailing him to at least 80(I like him anyways).
Arash, for farming purposes(only need one just for some extra stats)
Saint George so he tank harder, better, and longer
Euryale, because her anti male np is amazing in Camelot
David, Billy: All-around good archers anyways
If you haven't grailed Hans, do so. He needs the extra hp
Honestly, grails are for lower starrred servants to use because the stats given are linear, and it pretty much has little to no effect on gold servants. Grail waifus/husbandos, because that's why this game exists

Stheno. All the way to 100.

Chloe, she very good dmamage dealer, but she has low HP
Kintoki rider, incredible ST Rider, who generates pornographic amount of stars
Santa alter, tons of dmg on her NP5
Herc, he's 5* in disguise, 10/10/10 him, bond 10 and 2 grails
Martha Ruler, rulers benefit from grail on attack more than many classes, plus she's one hell of a offensive tank. And she gets more attack than Jeanne Ruler with just with one grail...

You got Jalter and seriously ask whom to grail?
Considering you want to grail for utility or improve your teambuild:
1. Jalter
2. Herc

I grailed Altera and maxed her skills cause she is a fucking monster considering her immensely high crit damage... Jalter is the most versatile single target servant in the game so she would be my main suggestion at the moment.
Since you have a 5-Star single target lancer and you also have herc theres no reason in grailing cu i'd say...

you are impressively short on assassins... i'd also suggest to wait for a single target assassin to join ur party and then spent your grails on him - if you dont like jalter maybe?
(maybe you get to draw jack or gramps - both would be a good option - gramps the even better one)

Grail Raikou, she's extremely good for both farming and boss slaying. You can use her everywhere without worrying about class advantage and she deals extra damage to a lot of servants and annoying monsters. Really, there's no offensive unit on this game as OP on the versatility department (except Skadi!Lancelot or Skadi!Dantes).

Heracles. Welfares. Rama.