Best to grail gameplay wise

Ishtar cos FGO is 90% farming

Of those 3 gameplay wise I'd say Ishtar, though if you grailed Archuria it might make her a more reliable boss killer for tough fights without class advantage.

For me it's Archuria. She is a potential solo unit due to her heal. Also it improves her DPS when grailed since she is capable of NP spam.

You could never go wrong with Ishtar though (even though she didn't grace me her presence until now).

First off, level 100 4* are still worse than level 90 5* in terms of stats and Martha (Ruler) isn't too good of a 4* to begin with due to her under-performing in comparison to other servants with class advantage.

Archuria gains 1067 ATK and 1390 HP at level 100 while Ishtar gains 1160 ATK and 1334 HP at level 100 so in terms of stats, Ishtar outweighs Archuria; however, Archuria is a better servant over all due to her still being the best ST NP spammer even in JP, while Ishtar is interchangeable servants such as Gil.

In conclusion, it depends on what support servants you own, if you have Tamamo, go for Archuria; if you have Merlin, go for Ishtar. If you have either none or all of these, decide it based on which team comp you prefer to play, Archuria for Arts teams and Ishtar for Buster-crit teams; both are viable but Merlin makes Buster-crit stronger gameplay-wise.

tactically best one is martha but waifully best one is ishtar

As said by Wei, FGO is a game where you spend most of the time farming so AOE servants are really useful. Personally, I own Ishtar and I grailed her to 100 cuz she's also my waifu and ever since, i've been using her for farming and she works out really well (even against non-sabers and berserkers!). So go for Ishtar! Hehe

Gameplay wise would be martha get her to level 90 and save all your grail for other lower star servant you like

I sort of regret grailing Martha Ruler to 90 cos most of the time I bring a class advantage servant rather than use a Ruler. But she is unique as a 4* Ruler so there's that.... Grailing 5* to 100 is generally extremely inefficient but Ishtar is decent especially for farming. Grail who you like really.

You could try Grailing Saint Martha Ruler for the CCC event comming this few months, cuz YOULL BE WRECKIN MOON CANCERS!!

Serious Talk: I prefer Archuria cuz in boss fights, Youll need every percentage of Damage in order to defeat the boss before it defeats your servants, and thats why it has to be Archuria.. if you grail Ishtar and only use her on Farming, yeah she can clear waves easily with her NP but id rather use Arash mostly on 1st waves... they are still the same but with Arash, you could still clear waves and switch him out cuz hes useless without His NP (and hes dead anyway) and make way for other servants to fire another NP like Ishtar to destroy MORE TOUGHER enemies... and thats where She Shines... like a Morning Stah!

For lvl 100, I'd say Ishtar. Both she and Arturia are farmers, but Ishtar is better at it, and is more useful in non-farming situations. Martha is a bit too niche to match the other two imo, she has decent defence but doesn't focus enough on it for her to be good at it, and instead focuses on her attack where she'd be outdamaged by anything with class advantage.

EDIT: lol just realised it was summer Artoria. In that case Artoria 100%.

Resilient and NP spam route grail for Squirtoria. For Challenge Quest and Anti-Bosses.
Easier farming and strong burst grail for Ishtar. For Multiple Servants/enemy bosses killing and Wave clearing.

For me, i choose Ishtar because we farm everydays and you can even use her all the time.