The value of stats vary depending on your current stats. Use tools such as wowanalyzer to evaluate your stat priority.
In general, no one stat is so much stronger than the others that you would sacrifice total stats to gain more of a specific one. So for the most part, prioritize item level (ignoring gems and leech, of course). If two items are equal item level choose the one with more of your preferred stat, and use gems, food and enchants with your preferred stat.
In dungeons, crit is your best stat, and mastery is your worst.
In raid, mastery is your highest HPS stat if your entire raid is stacked. However, haste is better HPS if your raid is spread, and contributes to DPS while mastery does not. Typically I’d recommend stacking haste. But if you only care about HPS, and your raid is very stacked, mastery will outperform haste.
Mastery (Mastery: Lightbringer): You do more healing to targets who are closer to you. If your raid is stacked for most of the fight, mastery is your best HPS stat. But if your raid is very spread out, it’s your worst. Additionally, mastery is the only stat that does not contribute to damage, so in dungeons or other situations where DPS is important, mastery loses value.
Versatility: Increases your damage and healing, while reducing your damage taken.
I recommend choosing your race based on the tools they give you, not the minor differences in output. You gain more value from an extra tool to help you deal with mechanics, movement, etc, than from a 0.0001% difference in throughput.
For Alliance, the best races are Lightforged Draenei and Dark Iron Dwarf. Lightforged Draenei’s Light’s Judgment does excellent AoE damage in dungeons, and Dark Iron Dwarf have an off GCD self-dispel that both removes harmful effects and buffs output.
For Horde, the best race for M+ is Blood Elf because Arcane Torrent gives them an AoE purge. In progression raiding, I prefer Zandalari Troll with Embrace of Gonk because I value the 5% movement speed for mechanics, survivability and output. However, both Blood Elf and Tauren are perfectly acceptable for progression raiding as well.
Ring: Select the “Accord” enchant with the stat of your choice, based on the discussion in the “Stat Priorities” section.
Select the +50 stat gem with the stat of your choice, based on the discussion in the “Stat Priorities” section.
Food: Select the food with the stat of your choice, based on the discussion in the “Stat Priorities” section.
Rune: Battle-Scarred Augment Rune. The new Rajani faction in Vale of Eternal Blossoms will sell a reusable Augment Rune for 50,000 gold once you are Exalted with them.
Potion: Prepot with Potion of Unbridled Fury in almost all cases. If an encounter begins with immediate damage, requiring healing, you may prepot Superior Battle Potion of Intellect instead. If you require mana during an encounter, use a Potion of Replenishment. Otherwise, use a Superior Battle Potion of Intellect to boost your healing, a Potion of Unbridled Fury to boost your damage, or a Skystep or Lightfoot Potion if needed for mechanics or to avoid death.
Healing Potion: Use Abyssal Healing Potion to save your life once per fight.