Most people recommend White Mage. Especially because its spells have intuitive names such as "Cure", whereas other healers call them other things that may not be obvious to you without reading the tooltip.

The others have just as simple damage rotations (which involves spamming 1 button most of the time), however, they can be more complicated.
  • Astrologian has a card system, where they play cards and many people say it has button bloat. Overall, that is not going to make it the easiest one to get into.
  • Scholar is a barrier healer and was known as the "technical" healer in the past. Although it's a very good healer, you have to combine different heals to be effective in some cases, position the fairie, manage aetherflow and all of that asks you to know what you're doing.
  • Sage is a functional copy of Scholar in most areas, with the biggest differences being no fairie to manage, not sacrificing damage for certain heals, a leap/gap closer, and buffs to its heal potency instead of a few of the regens and heals that Scholar has.