Who is the best mage for this build?

Delthea, if she's +spd, because the other ones have tomes that are arguably better than a bladetome.

Ishtar has great base kit so no point in using blade on her.
Delthea has nice supportive tome but can run blade tome quite nicely with +spd or +atk iv.
Ophelia has preference into one-shoting her opponents so her perf tome is better.
Do you have other candidates for this build? Do you have +spd Taltiu?
If anything you can switch a slot for deathblow 3 for one-shot mage archetype, +atk Delthea or Mae can make most use of it.

Bladetomes are TRASH compared to Ishtar tome.
I would say Ophelia or Delthea... Delthea maybe unless you need buff your other heroes.

Ignore Ishtar. Her base kit is great. No need to change her weapon.

Delthea and Ophelia have very similar offensive stats so it could go either way, but Delthea could stand to gain more than Ophelia could. Ophelia has great one-shot potential, while Delthea's base starting tome is a support tome. She's still fine with it, but she doesn't gain any benefit from it outside of its prf damage, but a decent buff going toward a Bladetome's damage can easily make up for that.

I agree with most of what people are saying. Strictly speaking, Ishtar is probably the best at that build because she's the fastest, but Mjoelnir is a bit too good for me to recommend giving her a blade tome.

Ophelia is on the slow side among blue infantry mages even though she's not exactly slow. Her big atk stat lends her to OHKO builds, which leaves Delthea to use the blade tome, and I do like it on her because fortify def will serve both for extra damage and to bolster her ability to survive a hit from DC, especially if you don't use fury.