Enchants, Consumables and Gems:

Consumables:

Food:   [Lavish Suramar Feast]  
[Lemon Herb Filet] is slightly stronger but the regents are only optainable during the Darkmoon Faire.
Flask:   [Flask of the Seventh Demon]
Potion:   [Potion of Prolonged Power]
Rune:   [Defiled Augment Rune] (or the unlimited version   [Lightforged Augment Rune])
Healing:   [Astral Healing Potion] or [Healthstone]
Utility:   [Avalanche Elixir] - can be used on avatar to prevent fall damage and deal some additional damage to the boss
Stealth:  [Draenic Invisibility Potion] - usually not used as rogue because it shares cooldown with combat potions like  [Potion of Prolonged Power]

Enchants:

Enchants can vary depending on your gear and the scenario.
The general aim is to get Mastery > Versatility > Critical Strike > Haste
It is recommended to run a simulation to determine the best enchants for your gear.

Ring:   [Binding of Mastery] /  [Binding of Versatility] /  [Binding of Critical Strike]
Neck:   [Mark of the Trained Soldier]
Back:   [Binding of Agility]

Gems:

As with enchants, it is recommended to run a simulation to determine the best gems to use in your gear.

First slot:   [Saber's Eye of Agility]
Every additional slot:  [Masterful Argulite] /  [Versatile Labradorite] /  [Deadly Deep Chemirine]

Relics

Relics offer optimization to your artifact weapon and bring item level and one (two with 7.3) additional point in one of the minor artifact traits.
The power levels of certain traits depends on a lot of things including your gear, the boss, your Netherlight Crucible traits, and your guild's strategy.
The best way to evaluate relics is to run a simulation.

Trait	Recommendation
[Energetic Stabbing]	One of the strongest pure single-target traits (loses a lot of value as soon as any AoE is introduced to the fight).
[Weak Point]	Extremely potent with  [Insignia of Ravenholdt]  also good without [Insignia of Ravenholdt] on single-target fights.
[Soul Shadows]	Very strong with  [Denial of the Half-Giants] on single-target and multi-target fights.
[Gutripper]	Strong trait single-target and multi-target fights.
[Precision Strike]	Is a decent trait for single-target fights.
[The Quiet Knife]	An ok trait for single-target fights.
[Demon's Kiss]	One of the weaker traits with the changes to  [Nightblade]. Gains a bit of value with T19 2P on single-target fights.
[Fortune's Bite]	Worst DPS trait. Gets a bit stronger if you select  [Anticipation].

Netherlight crucible:

The Netherlight Crucible is a relic customization tool introduced in 7.3 that has 3 tiers. Tier one is always a flat item level increase for your relics, up to +15 per. Tier two has 1 of 6  random light and shadow traits you need to choose between.Tier three has 3   minor traits that can't be the same as the initial minor trait. You can only choose between 2 of them depending on what Tier 2 choice you made.
You need to have 60/63/66 points in your artifact weapon (8/11/14 points in   [Concordance of the Legionfall] )to unlock the Tier 2 traits, and 69/72/75 points in your artifact weapon to unlock the Tier 3 traits.

Tier 2 Traits:

Shadow Trait	Light Trait
[Master of Shadows] +2.8%	[Light Speed] +1.5%
[Murderous Intent] +2.8%	[Refractive Shell]
[Shadowbind] +4.2%	[Shocklight] +1.8%
[Chaotic Darkness] +3.8%	[Secure in the Light] +4.0%
[Torment the Weak] +4.4%	[Infusion of Light] +4.0%
[Dark Sorrows] +1.4	[Light's Embrace]

Legendaries

The strength of a legendary item depends on your gear, your available replacements for legendaries, as well as the specific situation.
The so-called "Best in Slot" legendary combination can therefore be weaker for your current situation due to equipped relics and/or your equipped gear.
Simulations can be used to get a basic knowledge of each legendary's strength relative to each other.
The best way to find the best legendary combination is to run a simulation.
That being said, typically the current strongest combination for single-target fights is  [Mantle of the Master Assassin] and  [The First of the Dead].

Legendary
[Denial of the Half-Giants]	Strong legendary only in combination with  [Convergence of Fates]
[Mantle of the Master Assassin]	Best legendary for the Spec that turns  [Vanish] into an strong dps cooldown..
[Soul of the Shadowblade]	Strong choice in AoE fights. Also makes the spec play a bit more smoothly.
[Kil'jaeden's Burning Wish]	Decent trinket for mythic plus or on fights that need burst aoe.
[Shadow Satyr's Walk]	Strong Single target choice that requires some practice for optimal use.
[The Dreadlord's Deceit]	Strong in certain burst AoE situations because we can line it up with our damage-increasing cooldowns.
[Cinidaria, the Symbiote]	An underrated legendary that is strong due to our high initial burst.
[The First of the Dead]	Our strongest single target legendary.
[Insignia of Ravenholdt]	Decent single target legendary that also works well with  [Gloomblade], [Weak Point] Relics and Critical Strike rating.

Tier Pieces

Every raid tier offers 6 set pieces that, when you wear 2 or 4 of them, give you an additional damage boost by enhancing your gameplay in one way or another. In Legion we are able to wear multiple tier pieces but are reliant on the titanforging system if we want to keep old ones. The table below shows the strengh of the tier bonuses without counting in item level or stat distribution on the pieces.
Certain Legendaries also limit our options to fewer pieces and may result in scenarios where we need to use a Tier piece that has sub-optimal attributes.

Bracers + Mantle	Dfa	MfD
T19 2P	3.7 %	4.9 %
T19 4p	1.6 %	1.4 %
T20 2P	5.1 %	4.2 %
T20 4p	7.9 %	3.6 %
T21 2p	5.9 %	4.2%
T21 4p	3.4 %	4.1 %

Hands + Mantle	Dfa	MfD
T19 2P	3.8 %	4.2 %
T19 4p	1.8 %	1.9 %
T20 2P	5.5 %	6.4 %
T20 4p	10.4 %	3.6 %
T21 2p	7.8 %	5.9 %
T21 4p	3.0 %	3.5 %

Hands impact the Value of T20 by a higher amount as you see from the chart above.

Each slot offers a different attribute budget which leads to some gear slots being stronger than others.
The general aim is to get the best possible stats with the tier pieces.

This chart can help you find the best tier pieces.

T21	Mastery	Versatility	Critical Strike	Haste
Head	2	-	1	-
Shoulders	-	1	-	2
Back	-	1	2	-
Chest	1	2	-	-
Hands	2	-	-	1
Legs	-	2	1	-

1... highest secondary attribute
2... lowest secondary attribute

T20	Mastery	Versatility	Critical Strike	Haste
Head	-	2	-	1
Shoulders	1	2	-	-
Back	-	-	1	2
Chest	2	-	1	-
Hands	-	1	2	-
Legs	1	-	-	2

1... highest secondary attribute
2... lowest secondary attribute

T19	Mastery	Versatility	Critical Strike	Haste
Head	1	-	2	-
Shoulders	-	-	1	2
Back	2	1	-	-
Chest	1	-	-	2
Hands	-	2	-	1
Legs	-	2	1	-

1... highest secondary attribute
2... lowest secondary attribute

Gearing up and attribute distribution

As you gear up, you will start to optimize your gear around your secondary attributes as much as possible while still using as many of your high item level pieces as possible.
Certain secondary attributes become stronger if you have low amounts of them. The best way to determine the impact each stat will make based on your current gear is to run a simulation.

Trinkets:

Currently, the strongest trinket  is  [Specter of Betrayal]. It is used alongside various other trinkets.
[Convergence of Fates] loses value if you aren't using   [Denial of the Half-Giants] , and as such usually gets replaced with higher item level Tomb of Sargeras trinkets, such as [Engine of Eradication].
If you use 2 trinkets with an on-use effect, the use of one will trigger a 10 second cooldown for the other one. Beware, on simulation the Trinket 1 will be used first, so always put the best one on Trinket1 slot, and if you're going to top gear in raidbot, lock trinket one, else it will be switched around and the simulation won't be acurate.
In this case, the rotation will need to be adjusted to account for the cooldown imposed on the second trinket.
The best way to evaluate Trinkets is to run a simulations but you can look at the graphs provided by  Ravenholdt TC or  Bloodmallet to get an overview.

Pantheon Trinkets:

Argus (last boss of Antorus) drops Trinkets that fall under the category Pantheon. There are 6 diffrent trinkets that get randomly assigned with push loot.
The drop chance is random and does not depend on the number of player in the raid.The drops are also independend, this means you can get 2 trinkets at the same kill.

What is push loot?
Push loot is loot that is automatically assigned to you after the kill(similar to legendary items). The loot configuaration has no impact on it.

Out of the 6 Trinkets there are 2 aviable for you:
[Golganneth's Vitality] - Agi Trinket
[Aman'Thul's Vision] - Wildcard Trinket

The pantheon Trinkets are restricted and allow you to only use 1 at the time.
[Aman'Thul's Vision] Does not count towards the legendary limit and starts at itemlvl 1000.
[Golganneth's Vitality] Can be upgraded with [Pantheon's Blessing] up to a itemlvl of 1000.
While both Trinkets are strong you benefit more from [Aman'Thul's Vision] due to the high itemlvl.
[Golganneth's Vitality] need to be upgraded 6+ times to be close/higher in value.

How does the empowerment work?
You need to have at minimum 4 player with the Raid with an Unique Trinket. [Aman'Thul's Vision] is a wildcard and counts as 1 unique trinket each.
The empowerment triggers if 4 procs are up at the same time, this means that the uptime of the empowerment will increase with progression in the raid and more raidmembers acquiring the trinket.

Interesting Items

Craftable Items as well as trinkets and mythic plus drops.

Item	Discription
[Swapblaster]	Can be used to skip certain areas in mythic plus.
[Failure Detection Pylon]	Can be used to save time after a wipe in mythic plus.
[Ultimate Gnomish Army Knife]	An additional item that can be used to resurrect a group member, however the chance of success isn't that high.
[Reaves Module: Failure Detection Mode]	Besides the misleading tooltip resurrects the nearest party member.
[Nitro Boosts]	Can be used as an additional movement cooldown. Shares cooldown with combat potions.
[Subtle Shadowruby Pendant]	Reduces the cooldown of your Sprint by 10 seconds. However, you need to make the killing blow on the mob.
[Stabilized Energy Pendant]	Increases your energy by 5%.
[Gnawed Thumb Ring]	Only increases magic damage.
[Memento of Angerboda]	Procs every time a party member dies. Strong choice in progression.
[Eye of Command]	Is a very strong stat stick with crit, requires a mouseover macro if you want to DoT another target without losing the stacks.
[Ring of Collapsing Futures]	Requires you to use the item every 30 seconds to get the greatest benefit. Debuff is removeable with  [Cloak of Shadows].
[Convergence of Fates]	Class trinket that impacts our [Shadow Blades].One of the best trinkets with a rppm of 9.
[Nightblooming Frond]	Has a low proc chance and is therefore not consistent in damage.(sims right)
[Umbral Moonglaives]	Extremely potent AoE trinket. Requires the mobs to stay within its effect range to be efficient.
[Specter of Betrayal]	Strongest ST trinket but requires the boss to be relatively stationary to get the highest benefit.
[Engine of Eradication]	Has a very good proc that gets you a very high uptime if you collect all orbs.
[Vial of Ceaseless Toxins]	Is a strong trinket that offers you the option to abuse the reset if the fight allows it.
[Draught of Souls]	Makes your character immune to knockbacks during the channel.