I have three ilevel 200 epic rings. I always favoured Keystone Great-Ring to rule them all, until one of our guild MTs described it as "poooooo". Chastened, I consigned it to the bank, and used the other two. Recently I have begun to wonder if he was right. Here is my analysis:
Equip: Increases your dodge rating by 49 (1.25% @ L80)
Equip: Increases defense rating by 28 (5.69 @ L80).
Equip: Increases your dodge rating by 36 (0.91% @ L80).
Equip: Improves hit rating by 25 (0.76% @ L80).
Signet of the Accord
Equip: Increases defense rating by 31 (6.3 @ L80)
Equip: Increases your parry rating by 35 (0.71% @ L80)
Equip: Increases your expertise rating by 29 (3.54 @ L80)
Comparing KGR with DB
KGR: +490 armour, +0.1% chance to dodge
DB: +6 strength, +0.23% chance to parry or block, 0.76% chance to hit
Ignoring block chance and amount for now, it comes down to 490 armour versus 0.13% additional avoidance and some hit rating. KGR is better for EH, DB is ahead for avoidance, blocking and hit.
Comparing KGR with SotA
KGR: +490 armour, +22 health (assuming kings), +1% chance to dodge
SotA: +0.96% chance to parry, +0.25% chance to block, +4 strength, plus some expertise
Here, avoidance is broadly equal, so it's armour versus block chance/amount and expertise. I can't remember the exact figures, but I do know that expertise rating is usually twice as good as hit rating below the soft cap.
Ignoring the mean words of the guild MT, I will use KGR as having the higher effective health, and DB for the avoidance and the hit (I am paranoid about taunt resists, which are not governed by expertise). SotA will be useful for when I need to tweak my gear to include more threat.