Stat Priorities
In general, you want to have as many stats as possible, no secondary is worth dropping item level for. At equal item level in general you prefer intellect, mastery and crit while trying to avoid haste.
Stat Ranking
In Raids:
Intellect > Mastery >= Crit > Versatility > Haste
In Mythic+:
Intellect > Haste > Crit = Versatility > Mastery
Stat Outline
Intellect: Increases your spell power which boosts your abilities throughput.
Mastery: Echo of Light - a % of your direct healing is repeated over 6 seconds. Very powerful throughput when it does not overheal. It ticks every 3 seconds, and is NOT affected by haste.
Critical Strike: Increases your chance to crit. Mastery can’t crit by itself, but since the healing is based entirely on the healing done by the spell that proc’ed it, the HoT will be stronger if the initial heal is a crit.
Versatility: Flat out increases damage and healing done, while decreasing damage taken.
Haste: increases casting speed, reduces the global cooldown and reduces the cooldown of Prayer of Mending and Circle of Healing.
Races
Throughput wise, the races are all really close and you will never be forever behind because one chose one race over another.
Goblin does, however, deserve a special shoutout due to their racial offering some much-appreciated mobility.
On the Alliance side, Void Elves have a decent mobility racial as well, and as such are the premier option.
Enchants
Enchant weapon - Celestial Guidance
Enchant ring - Tenet of mastery
Enchant gloves - Shadowlands Gathering (optional)
Enchant chest - Eternal Bounds
Enchant Bracers - Eternal Intellect
Enchant Boots - Speed of Soul
Enchant Cloak - Fortified Leech or Fortified Avoidance
Gems
Intellect Gem: Leviathan's Eye of Intellect x1
Mastery Gem: Masterful Jewel Cluster
Consumables
Armor Kit: Heavy Desolate Armor Kit
Food: Iridescent Ravioli with Apple Sauce
Oil: Embalmer's Oil
Healing Potion: Spiritual Healing Potion
Potion: Potion of Sacrificial Anima (with 4x Prayer of Healing or 3x prayer of healing + 1x Holy Word: Sanctify)
Mana Potion: Potion of Spiritual Clarity
Flask: Spectral Flask of Power