Best use of Grails

There are two ideas you can go with when it comes to grail ascension. The most popular is to just grail servants you personally like, because the stat boost from grail ascension is not really all that huge. The other is to grail your good 4* servants to 90, because that's probably the most optimal use from a min-max perspective. The least optimal thing you can do is grail a 5* servant to 100, because past level 90 you only get 2 levels per grail instead of 5, but if the servant you like is a 5* then go for it.

And specifically, if you’re going to grail a 5*, don’t grail waver (unless you really, really like him). His utility comes from his skills, not his stats, so there’s little value to boosting him to level 100.

i have read some thread in reddit about which servant should be grailed. and it said that the best servant to grail is 1* - 3* servant not 5* servant. grail 4* servant is okay but grail 5* servant is foolish. 5* servants stat wont raise much like 1*-3* servant. Grail jalter from lvl 90 to 100 will raise her hp and attack around 1000 and you need 5 Grail. But if you grail arterios from 60 -90 you will raise his hp and attack around 2500 and you need 4 grail. i believe grail 1*-3* servant is the best choice but grail 5* for waifu purpose is also not bad choice
sory for bad english

Grailing 4 stars from 80-90 will be the most optimal decision. However to say it's the best use of your grails is wrong. If you got a top tier 5 star, especially if NP + then grailing them to 100 would be a good decision. I for one would highly recommend you to grail J alter to 100.

And I'm not really big on the 3* from 70-90. I personally think it's a waste of your grails unless you really like the character. First of all you are using 4 grails, and on top of that 3* are weak even when 90.

A bit late, but if you're looking for gameplay reasons then go with classes such as Lancer, Berserker, Avenger, and Ruler because they gain more from stat increases. However, grailing 4*'s are shown to be a lot more efficient due to getting 5 more max levels than the 2 you get from grailing 5*'s. At the same time, some 5*'s are worth to grail such as Cu Alter, Jalter, etc.

Main rule though: Grail who you like

If I had a Jalter I would grail her just to put her at the top of the list of support servants for friend points.

I like Arash... And now he is at:
Lv.92
Skill Lv 10/6/10
Max Fou Atk&Hp
Bond Lv.10 (got CE Bond of course)
NP Lv.5

And now still waiting for his Interlude '-'

All i want to say is... Grail whoever servant you like