Grailing for gameplay

You'll probably find most people here would say only grail for love (waifu/husbando). Most SSR do just fine in Lv90 and that extra 10 levels doesn't add that much.
So grail him if he is your all time favourite

I think one should Grail their favorite Characters myself, but if you want to grail for Gameplay I think Dante would be a good choice.

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To elaborate, from what I have heard on here, youtube, and reddit; there are two major strategies when grailing for gameplay

Strategy 1-Grail Effeciency:

The most efficient use of a Grail is to grail a low star character. The lower the maximum level the more additional levels each grail gives.

At max level 60 (a one star servant) you get 10 more levels.
At max level 65-85 a grail gives 5 more levels.
At max level 90-98 you only get 2 levels per grail. Level 100 is max.

Because of this and the fact that grail growth is linear, you will get a much greater stat boost per grail using it on someone like Spartacus rather than Jack the Ripper. Not only that, the cost involved is also much lower grailing a low rarity character.

Strategy 2: Grail for highest possible max stats

While grailing for maximum efficiency will give you the most stats for your grail it has one major problem: even with this boost a low rarity servant will still not have comparable stats to a higher rarity character.

Since level growth is linear Spartacus will still have inferiour stats to JTR at the same level. So, even if both are level 90 or 100, jack will outclass Spartacus in overall stats.

Because of this, in order to get the most powerful character possible you are going to want to Grail a 5* character.
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In addition to this, grailing for gameplay often involves considering the TYPE of character you want to grail and the stat gains you desire.

A support character, for instance, isn't considered as good of an investment for a grail as they don't do much damage and you are mainly grailing them for more hp. A DPS is considered a better investment as the increase in atk will improve their damage as will them living longer to do more damage. That said, if you are having a hard time keeping your support alive it might be worth the additional HP.

Further, because burster type cards get more out of attack stat than the other cards, burster servants supposedly get more of a boost from grailing than an arts or quick character. That said, if you run arts teams and with Skadi coming out in the future I think it can be argued that grailing an Arts or Quick character is not too bad of a choice.

Finally, you should consider class. An assassin or caster character have 0.9 damage modifiers meaning that they won't get as much out of the increased stats as a Lancer or Beserker would. So while JTR is a better choice for a grail than Spartacus, Cu Alter would probably be a better grailing choice than Jack. Of course, one might want to grail Jack over Cu to make up for her damage modifier.

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Dante being a 5* avenger means he should become pretty powerful and if you use him alot he will give you a great return on your investment. His only down side is being a quick and if you plan to get skadi when she comes out that would make Dante a good choice.

Of course this is all hearsay and my own personal opinion, so please take it with a grain of salt.

None of the 5-stars are really worth grailing for gameplay. The returns you get per grail become so low, not to mention it costs you 55 million QP, which is why people say to grail for love.

Before you Grail him, as a reminder DW can release another broken or strong servants as she/he wish.

Bonus :
Go play FGO JP first, try using your friend Achilles or lvl 80 welfare Rider Ishtar NP 5. If you can't raise his NP lvl to NP 2++ and can't afford your own Skadi, you better don't Grail Dantes at all.
The easier and more affordable or budget options, Grail lvl 80 Ishtar Rider to LVL 100. Her performance ? Imagine a lvl 100 NP 5 Crit Based servants with good survivability kit and high crit damage potential. If she's your waifu then it's a total win-win situations for you.
For more expensive options, Achilless lvl 100. He's basically 5* Rider Ishtar.

Why they're better as a grail candidates compared to Dantes ? Dantes relies heavily on Double Skadi, Achilles and Ishtar both is a monster, they're a natural Crit star magnet and also a Crit based servants, Dantes have no survivability, Ishtar and Achilless is a juggernaut blessed by DW with good offense and defense. Ishtar and Achilless can solo lots of stuff while Dantes can't do it.
Gameplay wise, Achilles and Ishtar can do TONS of STUFF that Dantes can't do.

Or you only have seen double Skadi + Dantes memes then, what about double Skadi + Ishtar or Achilles memes.
Skadi and another support can absorb lots of your Crit stars after Dantes use his NP or use another Quick cards, it would be a blunder if you can only afford a lvl 100 NP 1 Dantes without double Skadi.

Gameplay wise, Skadi can support a lot of Quick servants. No Skadi = no Dantes memes, general team ? Merlin and Waver or Hans can steal his stars, without Crit star magnet CEs he can't be a Crit servants.
Strongest in this game is support, like mentioned before no Skadi = no Quick Memes.
No Skadi = Achilless and Ishtar can do another solo or destroy things with their Crit damage, they're not really depend on support like Skadi in the first place. Give them a Merlin or Waver or Hans then enjoy your Crit based party.
See the differences ?

This post is for OP, before he grail Dantes for gameplay let him compare Dantes with another superior Quick AOE servants.
That's why just Grail for love because this game can be cleared by 1-3 stars servants. Grail for the servants you truly loves ? No regrets.

I've commented quite a few times about grail efficiency blah blah in other posts; putting that aside, I do have to say that Dantes is one of the better candidates for grail among 5*.

He has one of the, if not the most, highest np gain when attacked. That combined with the lack of immunity as well as avenger class affinity makes HP point more valuable to him than some others, comparatively.

Avenger also has passive 1.1 class mod, ergo you get a bit more atk points than you do when you grail.

I personally don't grail 5* because it costs a shit ton of money, and maybe because of my OCD, I don't want to stop at like, 96. It just looks ugly. If I am to grail a 5* servant, they get grailed to 100. There's no inbetween, making it an arduous process. But I digress.

While in general I'm not a fan of grailing to 100, there might be a reason (eventually) for Dantes: if you want to do the 3-in-a-row NP that he is known for in JP (post-Skadi), you need him to do as much damage as possible--NP 3 is considered the minimum. The extra ATK stat should help with getting the overkill required to charge his next one

If your Dantes is NP2 or more sure, but otherwise I wouldn't. You need higher NP levels to Double Skadi Dantes, so that's not really worth it.

Once your roster is filled out you'll find yourself using class-neutral servants much less. Why Bring Dantes against archers when you have a lancer? Why bring him against casters when you have a rider?

So unless you've got him at NP2+ (ideally 3 or 4) or totally love him, I wouldn't recommend it.