DreamgroveGenerally, with the exception of trinkets, you will just want to use whatever is the highest item level items you have available.
If you are dying, or taking too much damage, it is because you or members of your group are playing poorly. It is not because of your gear. If your DPS is too low, your gear will be more relevant but it will be a tiny factor compared to the play of again both you and other members of your group.
Gear
Survivability
For items with Agility, Armour, and Stamina on them, any item level upgrade will be an upgrade defensively. Sockets should be ignored when valuing pieces with those statsa.
For rings, if it has versatility and mastery, consider it to be +10 item levels. If it has versatility or mastery but not both, consider it to be +5. If it has versatility or mastery as the lower stat, consider it to be +5. Sockets are worth ~15 item levels on a ring.
DPS
For DPS upgrades, it is recommended that you sim your gear to determine what items will give you more DPS. In general, item level will almost always be better than any particular stat choice, because your secondaries are very similar in value.
Stats
Survivability
Armour (for most physical damage) >= Agility > Stamina > Versatility = Mastery > Haste > Crit
Armour from most of your gear is not the same as armour on trinkets or enchants. The former is increased by Ironfur, and the latter is not. Armour is extremely good at reducing physical damage taken. The vast majority of relevant damage when playing the game is physical. Armour does not generally reduce damage taken from bleeds, but there are some exceptions on a boss-by-boss basis (such as Harjatan from Tomb of Sargeras).
Agility grants Armour through Ironfur, and its defensive value depends almost entirely on how many stacks of Ironfur you can maintain. That being said, at one stack it is already stronger than Stamina (at raid-level gear), and at two stacks it completely blows Stamina (and Armour, for that matter) out of the water.
Stamina is excellent at increasing your Effective Health. Although this is not useful if you already know you are going to survive and merely want to reduce your damage taken, the caveat “if you already know you are going to survive” is not a useful one when discussing survivability. Stamina is excellent at helping make sure you do in fact actually survive, for example by giving healers more time to heal you, or ensuring you live against spike damage.
Versatility is good for its damage reduction and for increasing your healing done by Frenzied Regeneration. Its damage reduction component gets better the more you have. There are no ‘caps’.
Mastery (Mastery: Nature's Guardian) is similar to Versatility in that it increases your effective health. It does this by increasing your health and your healing received - which is in effect the same thing as reducing your damage taken. Mastery is slightly stronger than Versatility until you have 2025 total rating from both stats, after which Versatility is better.
Haste allows you to use your abilities faster (due to reducing the Global Cooldown and the cooldown of Mangle and Thrash, and your autoattacks), so you get more rage. It also reduces the cooldown of Frenzied Regeneration.
Crit gives nothing but dodge, which is an unreliable stat and the amount of it you get is very low per point of Crit. As such, it is worth almost nothing for survivability.
You cannot realistically sim yourself for survivability. The primary reason for this is that there are no particularly good metrics to measure it by. You can measure your “total external healing required to keep you alive” - the lower, the better - but a tank who has a higher total external healing required to be kept alive may still be a better tank than another, who needs less in total, because they are easier to heal, or survive more reliably. The metric “TMI” can be used to measure spikiness in health, but again this is not useful in isolation.
DPS
Agility > Haste >= Crit >= Versatility >= Mastery
Realistically, all of our secondaries are very close in value and will depend on how much you already have of each. They are all worse than Agility per point. If you are catweaving, Haste is far better than everything, including Agility. To determine which stats will give you the highest gain, you should sim your character on Raidbots.
Gems and Enchants
Gems
For survivability, use one Kraken's Eye of Agility, and the rest Masterful Tidal Amethyst (or Versatile Royal Quartz if you have a high amount of the two combined).
For DPS, sim yourself and use the gems corresponding to whatever your best stat is.
Rings
For survivability, use Pact of Mastery, or Pact of Versatility if you have a high amount of the two combined.
For DPS, sim yourself and use the gems corresponding to whatever your best stat is.
Weapon
For weapon enchants, you should use Enchant Weapon - Stalwart Navigation for survivability and the relevant secondary enchant following the rules outlined earlier for DPS.
Trinkets
The trinket selection from pre-raid is not outstanding. There are a few stat stick-type trinkets you may want to aim for, as they will provide the most consistent defensive and offensive boosts:
Azerite Gear
For a comprehensive look at what Azerite traits you should aim for, refer to the Azerite Analysis.