Mastery >= Crit > Versatility > Haste
In raids, you will want to prio Mastery, which is one insane source of healing of your kit.
Haste doesn’t scale that much with Holy’s healing (since it won’t affect your Mastery per se), so you want to avoid it healing-wise.
Haste > Crit >= Versatility > Mastery
The M+ build is aimed at improving your DPS & survivability, which are both very important in this kind of content.
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.
If you’re Horde, it’s highly preferred to pick Goblin, thanks to Rocket Jump. This 1 min 30 cooldown utility is pretty insane, even more, because of priests’ general lack of mobility.
If you’re Alliance, Void Elves have some mobility utility thanks to Spatial Rift that you will want to consider. Throughput will be very close between all races, so those utility picks are highly praised.
Enchant Weapon: Sophic Devotion
Enchant Chest: Waking Stats
Enchant Rings: Devotion of Mastery
Unique gem: Skillful Illimited Diamond
Healing Potion: Dreamwalker's Healing Potion
Flask: Phial of Elemental Chaos (default choice)
Other choices are Phial of Static Empowerment (it will require you to stay idle most of the time) or Phial of Charged Isolation (it will require you to be far from other people) ; both of these conditions are difficult to meet, and you probably won’t gain the full value of these phials.
Rune: Buzzing Rune