For raiding Venthyr is looking like the best option. Castle Nathria has plenty of cleave on many fights which only adds to the value of Condemn throughout a fight.
Condemn has an added defensive benefit as any primary target you hit with it will do less damage to you in a stacking debuff.
Kyrian does pull ahead on AoE with Spear of Bastion adding a good amount of burst AoE. Also adds another Rage generating ability to the kit which is nice for Arms, even if it is on a 1 min CD.
Soulbinds and Conduits
M+ Potency option: Crash the Ramparts. Only when paired with Dreadnaught. The extra Overpower damage is converted to extra Dreadnaught damage also. Crash the Ramparts likely has some single target potential also.
Additional mention to any relevant Covenant Potency Conduit as all of them provide good benefits to our Covenant abilities.
Stalwart Guardian. Reduces Die by the Sword’s starting from 20 seconds at Rank 1 and by a further 2 seconds with each additional rank. This is a fantastic Conduit for Arms which has historically been a very squishy DPS spec.
Inspiring Presence. Rallying Cry health increase and duration increased by 20% at rank 1 and a further 2% each with every additional rank. Rallying Cry is a very strong raid-wide cooldown. Further improving the effect is a great option for Arms Warrior.
Signet of Tormented Kings is a great legendary choice both on single target and AoE. Choosing this legendary will usually promote the use of the Avatar talent in order to proc it more often. Playing around this legendary, particularly on single-target, will be required.
Battlelord is a very solid choice for a single target legendary. It addresses one main concern of Arms Warrior, Rage problems. The proc rate is quite high but this legendary suffers during the Execute phase, particularly as Venthyr as you are likely to be playing Massacre with Condemn, leading to a 55% total “Execute phase” (100-80% and 35-0%).