Mastery, Haste and Crit are all relatively equal in value. You should try to go for the highest ilvl pieces you can while keeping these 3 stats in some sort of balance. Having 12k mastery and 3k haste is a bad balance for example, having 7.5k of each would be better.
Your stat weights also change a lot depending on the fight and which talents you use, this is why it's generally best to just go for ilvl, though I personally prefer mastery as the best stat because it gives the most value when people need the most healing through Cultivation and Germination.
There are some key haste breakpoints when using T19 4 Piece. Here is an imgur someone linked me a long time ago with all the haste points, it's useful to have bookmarked:
Legendaries vary in value depending on the fight and situation and most of them are very strong in the perfect situation. Here is a general overall list of how I rank them:
- Velens Future Sight
- Aman'Thul's Wisdom (With T19)
- Prydaz, Xavaric's Magnum Opus
- Soul of the Archdruid
- The Dark Titan's Advice
- Ekowraith, Creator of Worlds (Because of the insane amount of stats at 970 ilvl)
- Tearstone of Elune
- Chameleon Song (Comes new in 7.2.5, for farm would rank higher but on progression this insane rng requirement is not reliable)
- Edraith, Bonds of Aglaya
- Sephuz's Secret (Rank higher on regular dispel fights)
- Essence of Infusion
- X'oni's Caress
Trinkets & Effects
BIS trinkets for progression are:
Check out this post for information on the new trinkets coming in Tomb of Sargeras.
For Horde you can only play Troll or Tauren. Troll is the way to go 100%, pair Berserking with Tranquility for a faster Tranquility or with Innervate for more free spells, or with Essence of Ghanir/Flourish combos. Very versatile.
For Alliance you can go Worgen or Nightelf. Nightelf is the way to go for the passive stats!.
- Neck: Enchant Neck - Mark of the Claw
- Cloak: Enchant Cloak - Binding of Intellect
- Rings: Enchant Ring - Binding of Mastery/Haste
You should always use the following consumables to maximize your healing:
- Flask: Flask of the Whispered Pact
- Food: Lavish Suramar Feast
- Potion: Potion of Prolonged Power/Leytorrent Potion/Ancient Mana Potion