Essential legendaries:

Saruan’s Resolve:
Light/Hand of the Protector gains 1 additional charge and has 10% reduced cooldown.
Basically double’s your spot healing on yourself and your group. If you feel like you need the double HotP healing during any pull, use this.

Tyelca, Ferren Marcus’s Stature:
Avenger’s Shield deals 20% increased damage and jumps to 2 additional targets.
20% increased damage means 20% more absorb every time you click Avenger’s Shield + you hit 5 targets instead of 3 targets which, again, increases your absorb healing and your damage.

Sephuz’s Secret:
Gain 10% increased movement speed and 2% Haste. Applying a cc effect triggers a 25% haste buff and 70% movement speed for 10 seconds with a 30 second internal cooldown. This is one of the best offensive legendaries we can use on trash and on some bosses. You can trigger the effect with HoJ/Blinding Light/Repentance, Rebuke and Avenger’s Shield (if it silences or interrupts).

Soul of the Highlord(Holy Shield):
Increases your block chance by 15%, allows you to block spells, and your successful blocks deal 73,687 damage to your attacker.
15% block is great but the main reason you use this ring is to block magic abilities. This is especially useful on fights like Ymiron (first boss Maw of Souls) to mitigate the Dark Slash.

Pillars of Inmost Light:
Eye of Tyr deals 300% increased damage and has 25% reduced cooldown.
Use this when your aim is to get as much burst damage in a small window of time. This is purely a DPS legendary used on either short pulls or if you need to get out of melee after a short duration of time.

Situational legendaries:

Aggramar’s Stride:
Increases your movement speed by 75% of your Haste, Critical Strike, Mastery, or Versatility, whichever is highest.
To get the most out of the movement speed gain you need to stack one of the secondary stats as high as you can. This is mostly something you use for a “speed set” to get from pack to pack. Aggramar’s Stride could also potentially be used for a kiting pull. The idea behind this is so you can run in a circle, making sure all the enemies are somewhat grouped up whilst not getting hit by any auto attacks.

Uther’s guard:
Your Blessing of Freedom, Blessing of Sacrifice, and Blessing of Protection have 50% increased duration and heal the target for 15% of their maximum health.
The 15% heal is insignificant, but the 50% increased duration is very good in some specific fights.

Unused legendaries:

Breastplate of the Golden Val’kyr:
Each enemy hit by Avenger’s Shield reduces the remaining cooldown on Guardian of Ancient Kings by 3 seconds.
The cooldown reduction you gain on Guardian of the Ancient Kings is not significant enough to out value other legendaries.

Chain of Thrayn:
Avenging Wrath increases healing done by an additional 50% and increases damage done by an additional 10%.
During Avenging Wrath your HotP is already a massive heal. Getting an additional 50% is pure overkill. The 10% damage increase is out valued by Sephuz’s Secret, Pillars of Inmost Light and Tyelca, Ferren Marcus’s Stature.

Heathcliff’s Immortality:
You take 4% less damage while standing in your Consecration.
4% damage reduction doesn’t come close to the value of the essential legendaries.

Archimonde’s Hatred Reborn:
Gain an absorb shield for 30% of your maximum health for 10 seconds. When the shield is consumed or expires, 75% of the damage absorbed is dealt to nearby enemies, split evenly. 1 min 15 seconds cooldown.
Consider this a extra cooldown, which can be good in certain situations to avoid a 1 shot mechanic. However, you should always have one or more cooldowns up to deal with 1 shotting mechanics. The damage gain is very minimal compared to other legendaries.

Legendary combinations:

Damage:
Pillars of Inmost Light + Tyelca, Ferren Marcus’s Stature is the best single target damage output combination we have, assuming you are not able to proc Sephuz’s Secret in which case you would switch out Pillars of Inmost Light with Sephuz’s Secret.

Mixed damage and survivability:
Tyelca, Ferren Marcus’s Stature + Saruan’s Resolve gives you a very nice balance of damage and self sustain. Use this combination if you’re not comfortable going full damage.

Survivability:
Saruan’s Resolve + Soul of the Highlord is a very low damage output but the best damage reduction and self sustain combination we have for single target. Use this if you’re certain you could potentially die.
For AoE Tyelca, Ferren Marcu’s Stature out values Soul of the Highlord because of the trait Bulwark of Order.