Rauðrblade+

Either the fastest hero (Katarina) or the one with access to best buffs (Leo)

I think Katarina has the most Blade-Mage-like stats. You can pick Leo if you have a fitting horse team for him, but Katarina would be generally more flexible and efficient with it in all kinds of teams. Leo is more a Defensive unit imo.
The others already have perfectly fine working legendary tomes, so I wouldnt give it to any of them.

I think that Katarina or (perhaps) M!Morgan is the best candidate.

All the other heroes aside from the above have a really cool niche: Arvis has his debuffing potential, Lyon is a good Rauorraven (through his Legendary tome) and Triangle Adept user, and Leo is a great ranged tank with his new refine.

Meanwhile, Katarina doesn't really have the bulk to take advantage of the Raourowl tome, as anything physical is pretty much going to kill/severely cripple her. She's better off trying to kill things in one hit.

M!Morgan has a tome that works somewhat similar to Sothe's Peshkatz, so you might want to use him for that; but his offensive stat spread is slightly better than Katarina's (same Spd as her, but with 1 extra Atk) and he can't really perform any other niche too well, so you can consider him as another candidate. He's also easier to pull and 5-Star +10 than anyone else in case you want to make him a permanent member of your core, so that could be a justification for the investment.

I'd say Leo as a class buffs are magnificent with blade tomes. Haloween Nowi also makes good use so if you're planning on pulling reds in the legendary banner bare her in mind.

Arvis, Lyon, and Morgan already have good tomes in their base kit and Brynhildr is kinda lacklustre even with the new update but i think he can tank reinhardt now if he had deflect magic.

Owltomes are really good if you have the unit ally supported with someone and it'll be hard to get significant buffs on infantry units without having to worry about positioning too much unless you have a tactics team.

Leo is your best choice but if you have any other low tier red mage, a bladetome can easily make them viable