Int > Haste > Crit > Int > Vers > Mastery
Haste: Decreases the cast time of all your spells and increases the rate your dots tick at. Super good for multidot fights and generally gives you the ability to fit more damage spells into any given Atonement window make haste our best stat for raiding as a whole.
Critical Strike: Nothing special about it, just tuned decently.
Versatility: Flat damage / healing increase, doesn’t double dip. Also reduces the damage you take slightly.
Mastery: Increases your healing or absorption on targets that have Atonement on them. This does apply to Atonement but generally you don’t want to think about it too much, sometimes refreshing a Power Word: Shield at the very end of an Atonement is worth it. Mastery provides no damage.
The reality is all stats tend to be pretty close for HPS contribution and you’ll generally just take higher ilevel. I value Mastery the lowest because of no DPS/DR contribution. More constant damage or multiple targets to dot with Purge the Wicked or Shadow Word: Pain will make haste even better.
For healers in general, the normal throughput increase that DPS players should min-max when considering passives is mostly irrelevant, as they will usually not have any noticeable impact on your healing, although they do contribute to DPS. The most important thing is the active. Having either mobility increases like Goblin's Rocket Jump or Void Elf's Spatial Rift is super useful for priests as we lack mobility in general, and powerful actives like Troll's Berserking and Dark Iron Dwarves' Fireblood can augment our already powerful burst healing. The extra mana from Blood Elves should be considered too. Note that pure damage racials such as Undead's Touch of the Grave as of Battle for Azeroth no longer contributes to Atonement healing.
Goblin > Troll > Blood Elf > Pandaren > Mag'har = Tauren = Undead = Nightborne = Zandalari
Void elf > Dark Iron Dwarf > Worgen > Night Elf > Pandaren > Human = Dwarf = Gnome = Draenei = Lightforged Draenei = Kul Tiran
We will basically always just fully enchant for Haste, as its our best stat.
Rings: Enchant Ring – Pact of Haste
Just like with enchants, we are gemming pure Haste. Haste is better than Intellect for us, so we won't be using an epic Intellect gem.
Haste gem: Quick Owlseye
You should be pre-potting with a Battle Potion of Intellect to provide extra damage and potential healing in the first 25 seconds of the fight. Your flask will just be Flask of Endless Fathoms, and your food should be the one that gives the most stats since all are almost equal, which would be a raid feast such as the Bountiful Captain's Feast.
For your 2nd pot of the fight, you can either use another Battle Potion of Intellect for more throughput if you don't need the mana, or use a Potion of Replenishment to channel for 25000 mana, or if you don't have time to channel one you can use an instant Coastal Mana Potion. Making good use of health pots as an extra buffer between you and death can be very useful since priests are relatively fragile compared to most other classes. In this case use a Coastal Healing Potion.
Flask: Flask of Endless Fathoms