Int > Mastery = Critical Strike > Versatility > Haste
Intellect: is your primary stat. It will provide you with Spell Power which will increase the efficiency of your heals.
Mastery (Mastery: Echo of Light): your direct healing spells heal for an additional amount over 6 seconds.
Critical Strikes: double the effects of your healing.
Haste: lowers the cast time on your spells, as well as reducing your global cooldown.
Versatility: increases your damage done, healing done and decreases your damage taken.
A big portion of your healing is going to be from your Mastery: Echo of Light, which is why you want to get as much as possible. It is the most mana efficient stat by a large amount.
For Mythic+ you could go for Crit/Haste as you can basically drink as often as you want.
Goblin > Troll > Blood Elf > Pandaren > Maghar > Tauren > Undead > Nightborne > Zandalari.
Void Elf > Dark Iron Dwarf > Worgen > Night Elf > Pandaren > Human > Dwarf > Gnome > Draenei > Lightforged Draenei > Kul Tiran
Rings: Pact of Mastery
Weapon: Masterful Navigation
Potion: Battle Potion of Intellect
Flask: Flask of the Endless Fathoms