Who should I grail?

jalter and lancelot(saber) i think
then scathach(avengers are usually better to grail than lancers)

but the most important thing is :give your grails to your waifu/husbando

Grailed Shishou back when she came out for waifu reasons, didn't have any reason to regret it, since, in a quick team, she hits like a truck and her NP deals tons of damage too! So I can attest that a grailed Scathach works wonders :D

Do not grail Scathach to 100. She is really hurt by the upcoming break bars. Lancelot Saber is fine. Emiya is ok as well. It is unnecessary to grail Jalter to 100 for similar reasons as Scathach. In the end I am advising from a gameplay standpoint, and you should grail servants you like using.

Saberlot is amazing with a couple of grails. Yours ought to be even more so at np3, mine is np2.

I'm not really in favor of grailing 5*s myself, feels like a waste of a very limited resource.

I’m grailing Kiyohime, so yeah, it doesn’t really matter who you grail. Grail for love, not stats.