Let's start with the basics

Stat Priority

People tend to overthink secondary stats a lot when choosing gear. It's important to remember that item level is always king due to how strong main stat is. 9 times out of 10 you should be choosing your highest item level gear, regardless of what secondary stats it has. My rough rule for Leech/Avoidance/Sockets is that they're generally worth ~6 item levels. Below you'll find a breakdown of our stat priority for Mythic+ and raid.

Mythic+ Stat Priority
Intellect
Haste
Crit Strike / Versatility
Mastery
Main stat is incredibly strong this expansion and continues to be the dominant choice when gearing. This is unlikely to change for the rest of The War Within so make sure you're focusing on item level first!
Haste is our best and most important secondary stat as you need a base level of Haste for this spec to play well due to the cooldowns on our core rotational abilities and reliance on Holy Power generation. There are no haste breakpoints for Hpal, but higher haste means more mana strains so I recommend staying around 28-33% for season 2.
Crit lost some value due to our lessened need for hard casting in TWW, but still provides good throughput, mana efficiency and spot healing. Versatility is another great stat for Mythic+ as it increases both our throughput AND our damage reduction, which is vital for pushing keys. You'll want to get as high as you can with your these stats after reaching a solid haste level.
If you just want raw healing throughput, Mastery is your best choice, but not by a significant margin. Remember it also has no effect on our damage, which in Mythic+ is a significant part of what we do. It's a much better stat now with the return of the Beacon of the Lightbringer talent, but in general is still the weakest M+ stat.
Raid Stat Priority
Intellect
Haste
Mastery
Crit Strike / Versatility
Just like Mythic+, main stat is by far the most dominant choice when gearing, even moreso than Mythic+ because in raid there is no such thing as a bad secondary stat. Prioritize leveling up your gear before worrying about optimizing secondaries.
Haste remains one of our strongest stats in all content, taking priority even over everything until you have at least 20%. Just like Mythic+, I recommend sitting somewhere in the range of 28-33% so the spec plays well but you don't strain your mana too much.
Mastery is a fantastic stat for raw healing power, especially with the return of Beacon of the Lightbringer, but it still takes a slight backseat to Haste even in raid. Use this as your stacking stat once you've hit your desired Haste levels.
Crit continues to be a strong secondary in state, even if it's slightly behind Mastery for throughput. Versatility is a solid throughput stat in raid as it will give you more healing, damage and survivability.
Liberation of Undermine Tier Set

For the second season of The War Within, Liberation of Undermine is bringing us a tier set that focuses on Divine Toll and provides some nice safety healing via the Insurance! HoT all healers are getting from their 2-set:

2 Set: Your healing has a chance to apply Insurance! (a HoT) that is consumed and heals your ally when they drop below 40% health.

4 Set: Divine Toll applies Insurance! to allies it heals for 8 sec. Divine Toll's cooldown is reduced by 1 sec each time you spend Holy Power.

This tier set will have a major impact on our playstyle in Mythic+ but a relatively minor one in raid, placing more importance on using Divine Toll and building/spending Holy Power. It's more important then ever that you use Divine Toll aggressively this season in raid and never sit on it for more than 10 or so seconds. When it comes to Mythic+, the key to our healing here comes from building and spending fast enough that we can get Divine Toll down to a 30 second cooldown, allowing us to use it with every other cast of Beacon of Virtue. Alternating each virtue cast with either DT or Holy Prism will provide us with huge AoE and spot healing bursts every 15 seconds. As for insurance, I wouldn't recommend putting any thought into tracking it, just enjoy the free passive healing and safety!

Cyrce's Circlet

This customizable ring introduced in patch 11.0.7 will still be your BIS ring up until you get at least a 672 ilvl ring in Season 2. These three gems: Stormbringer's Runed Citrine, Fathomdweller's Runed Citrine, and Windsinger's Runed Citrine will be your BIS combo for all content.

Best Embellishments
Embellishments are a relatively small increase to your throughput, so don't stress too much over them. For START OF SEASON crafts, I usually recommend going 1h weapon -> shield, with Ascension on the weapon and Writhing Armor Band on the shield. This will give you the strongest early season boost regardless of what content you're doing, plus you can always strip the embellishments off your weapon/shield once you start crafting binding of binding or other embellishments so they aren't wasted. Below is a list of our best embellishments and which ones you should prioritize dependent on the content you're doing. For a chart of ALL embellishment values, check out QELive values and toggle between Raid or Dungeon on the top right of the header dependent on which list you're interested in seeing.
Best for Mythic+
Your spells have a chance to proc bonus stats for allies
While this won't provide much healing, it will bring significantly more damage than any other embellishment by a large margin so it's a fantastic pickup for Mythic+. None of the more healing focused embellishments will have a significant enough impact on your healing in keys to warrant running (the good ones we use in raid drop stacks out of combat, making it bad for dungeons).
Your heals amplify an allies gemstones
Similar to Binding of Binding, this is also focused on buffing your dps friends instead of yourself, and is only slightly behind the ring for damage in keys. Once again I do not recommend a healing focused embellishment as none of them contribute a meaningful amount of healing in Mythic+.
Best for Raid
Gain a stacking secondary stat while staying in combat
Ascension is our best option for raw throughput in raid, gaining stacks the longer you're in combat. This means that fights with heavy damage output early on won't see as much value as longer fights, but regardless of fight length this should always be your best option for throughput. I highly recommend crafting this on a weapon as your first embellishment of the season.
Doubles the effects of your Ascension embellishment
This embellishment doubles the value of your Darkmoon Sigil: Ascension embellishment, so throw this on one of your armor slots and enjoy doubling your weapon embellishment value!
Best Trinkets
Below is a list of the best trinkets you can get in the first season of The War Within, as well as the most powerful trinket combos for each content type. As it is early in the tier, make sure to check back here for updates to trinket lists if tuning happens. For a chart of ALL trinket values, check out QELive values and toggle between Raid or Dungeon on the top right of the header dependent on which list you're interested in seeing.
Best for M+ Pugging
Drops from: Liberation of Undermine (Lockenstock)
Healing procs that overflow to damage
This is a simple trinket that you don't have to put much thought into. It will randomly proc healing on allies (don't worry about tracking this), and any overhealing will spill into damage, making it an extra strong trinket choice for Mythic+.
Drops from: Liberation of Undermine (Mug'Zee)
Stat stick that increases crit based on APM
There isn't much to talk about this, it's simply a stat stick that grants you crit when it procs, and increases that crit with each spell cast. This synergizes really well with Hpal in keys as we have such high APM that you can stack up the crit very quickly.
Best for M+ Pushing
Drops from: Liberation of Undermine (Lockenstock)
Healing procs that overflow to damage
For the first time in a while, the trinket recommendations will be the same for pugging and pushing keys. However, if you're looking to min max your damage you may want to look into Bursting Lightshard, the trinket listed below.
Drops from: Liberation of Undermine (Mug'Zee)
Stat stick that increases crit based on APM
There isn't much to talk about this, it's simply a stat stick that grants you crit when it procs, and increases that crit with each spell cast. This synergizes really well with Hpal in keys as we have such high APM that you can stack up the crit very quickly.
Best for Raid
Drops from: Liberation of Undermine (Gallywix)
Mastery stat stick with some flavor
Our BIS trinket by a fair margin in raid is Eye of Kezan, this trinket isn't very unique but it is a ton of strong stats. Its value drops off significantly in Mythic+ due to it being a Mastery stat stick, which is our worst stat in keys.
Drops from: Liberation of Undermine (Sprocketmonger Lockenstock)
Healing procs that overflow to damage
As strong as this trinket is for Mythic+, it's just as good in raid. The damage portion of it may not be quite as relavent on farm, but for prog this will be a very strong pickup. Once again, I do not recommend tracking the healing buff this provides.
Other Trinket Options
Drops from: Priory of the Sacred Flame (Prioress Murrpray)
2m Use: Heal a Lightshard to deal damage
This trinket synergizes extremely well with Holy Paladin in Mythic+ (THIS IS NOT A RAID TRINKET), but it can be a little awkward to get used to. The idea here is to always line up this trinket cast with your Wings, which also means you should be using your Blessing of Autumn on other players so you don't desync your wings and trinket. Use it with Wings and place it in melee (you can macro it to just drop at your feet), then spam heal it for 20 seconds until it disappears and you'll do a huge amount of AoE damage.
Drops from: Liberation of Undermine (Rik Reverb)
Haste stat stick with flavor
It's unlikely you'll get one of these early on in the tier as they're highly coveted by DPS players. That being said it's a great healer trinket still if you can get your hands on one and not one of our better options.
Drops from: Priory of the Sacred Flame (Dailcry)
2m Use: Grants you secondary stats and some to allies
Signet gets stronger with how many people use it in your group, and it adds a ton of power to your wings window. More coordinated groups will likely run this for the extra stats.
Drops from: Liberation of Undermine (One-Armed Bandit)
Mastery stat stick with flavor
Not really something I'd suggest running in M+ but definitely a solid pickup for raid healing. If you can't get a better option, this will be a good option early in the tier.
Drops from: Darkflame Cleft (Blazikon)
Vers stat stick with flavor
This is another set and forget trinket which does provide some extra survivability which is always welcomed in Mythic+. It's also decent in raid and farmable via M+ so expect to see a lot of people running it at the start of the season.
Enchants and Consumables

With The War Within you'll see the return of many familiar enchants, oils and armor kits as well as interesting new ones! Before every dungeon or raid boss you'll need to make sure you have everything reapplied to your armor so I suggest getting a weakaura like this as a reminder.

Item Enchants
Chance to increase your Intellect or heal nearby allies
Radiant Power will generally be your best option for Mythic+ while Fiery Resolve will be the go to raid enchant. If you want one that works for both content types I'd suggest Radiant Power as Fiery Resolve loses a lot of value in Mythic+ but Radiant Power is still fairly good in raid.
Increases your Intellect by 745
Not much to see here, main stat is king as always so don't worry about the mana enchant option.
Increase your Haste by 315 / Versatility by 390 (-110 Mastery)
Your default for raid should always be Radiant Haste (or crit depending on your gear), but in Mythic+ I recommend Cursed Versatility as you can gain more desirable stats at the cost of Mastery which has very low value in dungeons.
Increase your Leech by 1020
Leech has always been our strongest throughput tertiary stat so your default cloak enchant should be Leeching Fangs, only swapping to Avoidance in very high keys if you need it to survive one shot mechanics.
Armored Leech (Bracers)
Increase your Leech by 2040
For the same reasons you want Leeching Fangs for your cloak, you'll want Armored Leech for your bracers in most cases.
Increase your Stamina by 131
Boot enchants are never very interesting, enjoy your free Stamina!
Increases your Intellect by 747 and Stamina by 230
In most cases having the increased health pool is always better so you'll default to this but if you ever find yourself running into significant mana issues you can try Daybreak Spellthread.
Weapon Runes & Gems
Increases your Crit and Haste by 155 for 2 hours
This expansions weapon oil happens to provide us with two very strong secondary stats so make sure you stock up! This will need to be reapplied every hour. Note: If you are playing Lightsmith, you should run Rite of Sanctification instead (you unfortauntely can't use both).
Increases Intellect by 181 and Movespeed per gem
As our main unique gem, it should come as no surprise we want more mobility! Reminder to equip at least one of each type of gem (sapphire, ruby, onyx and emerald) to get full value from this.
Increases Haste by 147 and Crit by 49
This is just our basic filler gem, change this out if you want more Haste/Vers/Mastery instead.
Phials & Food
Increases your Crit / Haste by 3323
Depending on which one your gear is lacking in more, you'll want to run either the Haste flask or the Crit flask.
Increases your Intellect by 445
If you have luxurious friends they may bestow upon you a grand feast, which is by far our best option for food. But for the common folk, we must dine on Jester's Board instead.
Increases your highest secondary stat by 445
This is the main personal food of The War Within, providing a large amount of secondary stat. If you don't have rich friends throwing feasts at you this will be your bread and butter.
Potions
Increases 3 Secondary Stats by 2617 for 30 seconds
Tempered Potion is the main throughput option this season but it's a bit different from previous expansions. Instead of granting main stat, it grants three secondary stats instead.
Instantly restores 4,000,000 health
As always, make sure you use your healing potions whenever you're in danger. You'll find these potions to be extremely strong self healing and off the GCD.
Restores 375,000 mana over 10 seconds (channeled)
In raid this should be your go to mana potion as it restores more than the regular one. Make sure you find a safe time and place to drink as you can't move once you start channeling!
Instantly restores 270,000 mana
This is the standard mana potion we get every expansion. Great for if you're in a pinch and need mana quickly and there's no time or safe spot to drink a Slumbering Soul Serum.
Increases up to 5 allies Intellect by 632
For my min/maxer friends, this is a unique potion that you can use right before the start of a key as your team will keep the main stat and you can use your personal pot while it's still active.
That's it for character setup, now it's time to dive into the playstyle of Holy Paladin.