Best Raven IV?

So, at +10, with LnD 6(not a big fan of fury raising a stat that is 23 base) he hits 64/52 atk/speed with a boon in speed and 67/49 with atk boon, or -2 each for fury builds. Since the playstyle raven drives for a galeforce nuke set, running +atk seems a little redundant since he will probably always proc heavy blade as it is, so I'd say 52 speed is a consistant place to double plenty of threats with a boon.
+Spd for me -Res
The other argument is that 52 speed wont outspeed demons, so might as well focus on nuking the armour's you'll always outspeed as is

Speed is arguably Raven's most important stat when you're building him up with the Life and Death refine of Basilikos. More speed is more reliable doubles, which is more units that''ll most likely get eviscerated by Desperation.

With Fury you should go with Spd boon. Since Linus has it and is a free unit, I'd also recommend giving Raven Brazen Atk/Spd, which puts him at an even 50 spd with a boon and L&D refine before buffs and merges, with the benefit of also raising his atk to 62, all for no additional defense drop

Depends on the context. If you want a full out glass cannon that hits hard and fast, going +spd with LnD 6 gives you a 60/48, +atk gives 63/45, and this is considering an unmerged one, so with 3 it would be 61/49 or 64/46. If you run heavy blade seal, you can fire off draconic aura in a single round while in desperation mode, or run Luna for deadly strikes. Unless you are putting him against speedy sword users or are facing super speedy builds/maxed foes, you should be fine with a +atk build. If you plan to try to defy the triangle, speed may be better so he can combat and kill faster sword users (but even 49 will not secure doubles against a +spd Ayra/Mia/Karla among others). Focus on nuking the bulky units, and use other units for the matchups you shouldn’t be in. He should be just fine with +atk.

But as Brenneko said, brazen atk/spd would be a much better skill to have in desperation range, as he could potentially take a hit in a situation and not die.

+spd is great to outspeed more units.
On my build (phoenixmaster build, to make something different than linus), +atk version would help to trigger heavy blade more easily.
I guess i would change the C slot with the atk version