Priest is a versatile caster that can deal damage, heal itself through almost any pull, and step straight into any dungeon group as the healer. In WoW Forever, leveling is smoother than ever: dungeon quests have been massively buffed, the new gear is loaded with Spirit and spell power, and every Priest now gets
Devouring Plague,
Fear Ward and
Shadow Word: Death baseline.
The fastest way to level isn't Shadow from level 10. A Discipline build that rushes
Meditation and
Penance, paired with a strong wand, kills quickly, rarely sits down to drink and saves a lot of gold. That matters, because your biggest goal on the way to 60 is a mount at level 40. At 40 you respec into Shadow for
Shadowform.
While leveling, open with
Mind Blast, apply
Shadow Word: Pain, add
Penance once you have it, and finish with your wand so the five-second rule refills your mana.