Smite priest is the single best levelling build in Vanilla>TBC>WoTLK expansions that relies on the Holy and Discipline talent trees in order to increase the effectiveness of holy damage spells and improve our mana regen and spellpower through spirit.
This is a hybrid build that can effectively deal damage to enemies while levelling and heal the party in dungeons. I don't suggest going to dungeons as a damage dealer with this build, at least early on, since your main weakness is running out of mana quickly during long fights and it's much better to have someone who can consistently deal damage in that DD slot.
I am not going to make a comparison with classic Shadow levelling builds YET, since shadow priests are gonna see somewhat major changes with the upcoming patch. Those changes can definitely increase the effectiveness of Shadow levelling, so I'll probably update this later.
This part is not important to the build, so generally you wanna choose a race that you like. However, races have different racial spells and bonuses and priests specifically also get additional unique spells depending on their race.
The unique priest racials are as following:
Dwarf Desperate Prayer Fear Ward
Human Desperate Prayer Feedback
Night Elf Starshards Elune'Grace
Troll Hex of Weakness Shadowguard
Undead Touch of Weakness Devouring Plague
Custom High Elf race also has priest racials, but I can't seem to find them on the database. Sun's Embrace gives reduced mana cost on next 5 Smite or Holy Fire casts while Divine Sanction increases your haste by 10% for a hefty mana cost.
I'd say PVE wise, Dwarf is the best race for Alliance priests due to Fear Ward, although the added Spirit from Human racial is good too.
On the Horde side, Trolls have Berserking which is great for burst.
As I said before, choose any race you like, none of them really matter for this build and the levelling process in general.
I. First talents we wanna get are Spirit Tap and Wand Specialization. They are both crucial to this and pretty much any other priest levelling build, so get both of them. Order is not important and depends on your preference: Spirit Tap for faster mana regen after killing a mob and Wand Specialization for increased damage from wands which are an absolute MUST for a priest. Seriously, as soon as you hit level 5, try to get a Lesser Magic Wand. From what I've seen, there are few custom quests that allow you to get other wands somewhat early too, but either way, just get your hands on one, they are an absolute blessing. All the way from 1 to 60, try to keep your wand up to date.
This is our talent tree at level 19: https://talents.turtle-wow.org/priest/5-0-U
Our general spell rotation is Smite>Shadow Word: Pain and we either keep Smiting or wand till the mob is dead.
You can also apply Power Word: Shield before pulling a mob to avoid damage and most importantly not get spell pushback.
II. Next, we wanna put points into Divine Specialization and Divine Fury. While getting additional 5% critical chance is not huge at early levels, because our base critical chance is low at this point, reducing the cast time of Smite and Holy Fire by 0,5 seconds is a must, since they can reach whopping 2,5 and 3,5 seconds. Instead of taking Divine Specialization right now, you can instead take Healing Focus and Improved Renew. We are going to get all three of these talents later on anyway, so choose the order for yourself.
This is our talent tree at level 29: https://talents.turtle-wow.org/priest/5-0UU-U
Once you unlock Holy Fire, your rotation should be: Holy Fire>Smite>Shadow Word: Pain. You can also Holy Fire>Shadow Word: Pain and then kite back until global cooldown is off and then cast Smite. There is no real difference, just do it the way you want.
III. Next talents we want to get are: Holy Nova, Improved Renew and Healing Focus/Divine Specialization. Afterwards we get Searing Light and Holy Reach that directly improve our main damaging spells. Regarding Holy Nova: I don't use this spell often, it costs a lot of mana and doesn't damage/heal much, but in my opinion it's way better to use that 1 skill point to unlock the next tier by getting a new spell than putting just 1 point into few other talents available here that wouldn't really improve your levelling efficiency in any way.
This is our talent tree at level 39: https://talents.turtle-wow.org/priest/5-FUV0CC-U
Our rotation at this point doesn't really change, but if you are having hard time getting a better wand, I suggest using PW: Shield and putting few more Smites before you start wanding after your initial rotation opener.
IV. Next, we unlock the best talent in the whole talent tree: Spiritual Guidance. It converts 25% of our Spirit into Spellpower. Might not seem like much, since our spirit at this stage is not that high, but because we prioritize Spirit stat, this talent lets us get a constant increase to spellpower, which affects both our damage and healing. On top of that, this talent synergizes with the Spirit Tap. Whenever you kill an enemy that is not grey, your spirit increases by 100% for 15 seconds which also affects your Spellpower, so chain killing actually gets more effective since you deal more damage during that period. You also regen mana at 50% rate while casting when Spirit Tap buff is up, so sustain is pretty good.
After getting Spiritual Guidance we can move to the Discipline talent tree. We wanna put points into Martyrdom, Improved Power Word: Fortitude and 1 point into Silent Resolve. Martyrdom occasionally allows us to spell cast without a pushback. Instead of taking Improved PW: Fortitude and Silent Resolve, you can take Improved Power Word: Shield, but in my opinion, having more health is better, since at this stage, we can already kill most mobs while our Shield is up, so there is no real need in improving it.
This is our build at level 49: https://talents.turtle-wow.org/priest/582-FUV0CC5-U
Once again, rotation doesn't change, but if you proc Martyrdom, feel free to Smite the enemy few times.
V. And finally, we finish the talent tree off with Inner Focus, Meditation, Silent Resolve and Improved Inner Fire. Inner Focus is a very useful spell with the variety of uses. Most of the time, you wanna use it before a powerful healing spell to cast it for free and with a higher chance of crit. From what I've seen, high elf priests also cast it before using one of their racial spells that is somewhat mana costly. It is also possible to only get your Silent Resolve to 4 and use that additional skill point for Reflective Shields talent. Since this guide is written before the patch, I don't know how good that talent is, but it sounds great.
This is our final build: https://talents.turtle-wow.org/priest/5W29I-FUV0CC5-U
At level 60 you can respec into specialized builds to slowly get into the raiding scene, but this build is still effective for questing, dungeoning and mob farming.
Stats you wanna prioritize are Spirit>Intelligence>Stamina. Items that give spellpower, spell damage or spell healing are also good. I suggest prioritizing items with high stats to those that give +shadow damage, because the only shadow damaging ability we use is Shadow Word: Pain. So unless you get an item with a really high bonus to shadow damage or simply don't have an alternative, take items with high spirit and intelligence.
Buying spells and upgrading them at trainers is painful. Seeing your hard earned gold coins disappear is heart wrenching. This is the reason why you should only buy and upgrade specific spells that directly affect your efficiency in levelling.
Btw, you unlock new spells/spell ranks every 2 levels, so level 2,4,6,8...
Must buy/upgrade spells are:
Smite, Holy Fire, Shadow Word: Pain, Power Word: Fortitude, Power Word: Shield, Inner Fire, Flash Heal*, Renew, Mind Control*, Prayer of Healing*, Cure Disease, Dispel Magic, Psychic Scream and Fade*.
Flash Heal* - in my opinion is the most reliable healing ability for the priest while levelling. It's fast, it's mana efficient and it does the job.
Mind Control* - should be optional, but the way I use it is mind control a mob inside a big pack, let the other mobs kill him, run away to reset the aggro. I do it in situations when there is a big pack of enemies that I can't handle alone.
Prayer of Healing* - good AoE heal I occasionally use in dungeons or when levelling with a party. Best used with Inner Focus ability to remove the mana cost. Not a necessary spell while levelling, but a useful one.
Fade* - allows you to drop all aggro which can save your life in dungeons. Mandatory if you dungeon fairly often, otherwise, optional.
Heal, Mind Soothe, Shackle Undead, Mind Vision, Mana Burn, Desperate Prayer.
All the optional spells have their uses, but don't really add much while levelling. The choice between Heal and Flash Heal is also a matter of preference. Priest heals with a longer cast time tend to be more efficient longterm, but I personally prefer a fast heal.
Hope this guide has been helpful! This is a first one I've ever made, so any sort of feedback regarding the contents and editing are welcome. I'm planning to expand on this, especially because of the upcoming patch that will bring some goodies to priests.