You generally want to prioritise item level. If you are comparing two pieces of similar item level, then you can follow the guidelines below, however:
- Stat priority can change based on how many stats you have already, for instance having a lot of crit will diminish crit value and having no mastery at all will increase mastery value slightly.
- Keep in mind the new Diminishing Returns for secondary stats added in Shadowlands to avoid getting the stat penalty (However, not likely during the first tier).
Rising Mist build:
Mastery value drops a bit lower compared to Upwelling build since Rising Mist HoT extension or Rising Mist healing itself is not affected by Mastery.
The difference between races is so minimal, that you should play what you like. If I had to pick a race, I would choose:
On Horde you could play Blood Elf or Pandaren.
Having Arcane Torrent and extra Crit as a Blood Elf, or having extra stats from food as a Pandaren is nice and might be a tiny bit more useful compared to other options. However, as I said, other races are by no means much worse or not viable.
On Alliance you could play Night Elf, Pandaren, Dwarf or Mechagnome.
Shadowmeld is a very nice ability to have, especially in Mythic+, and Dwarf and Mechagnome racials are good too.
Weapon: Celestial Guidance
Chest: Eternal Bounds
Bracers: Eternal Intellect
Cloak: Fortified Leech
Rings: Tenet of Critical Strike
Crit Gem: Deadly Jewel Cluster
Currently, it’s worth using 1 BfA Intellect gem, but this might get fixed.
Flask: Spectral Flask of Power
Oil: Embalmer’s Oil