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.
Night Fae does pull ahead on AoE with Ancient Aftershock adding a good amount of burst. The Korayn soulbind and the First Strike trait adds a regular crit buff upon first attacking a mob which plays very well into M+ specifically.
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.
Single target with AoE
This build is for single target with some AoE elements (mostly thinking for Council of Blood and potentially Sire Denathrius).
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. This legendary no longer suffers from Execute phase issues as of 9.0.5 as it now procs off of Overpower instead of Slam resulting in procs continuing to occur in Execute phases.
Enduring Blow has been buffed as of 9.0.5 making it the best choice for non-Venthyr Arms Warriors. With an increased proc chance and longer duration on the Colossus Smash applied, good RNG with this legendary can result in fairly competitive damage on pure single target fights.