Any of the healing jobs are good to start because they all have a similar healing kit but also have different tools to deal with the same situation, which means that, even if you know a fight as a WHM, you'll have some relearning to do when you switch.

However, I would say that the WHM is the easiest when it comes to learning how to deal with situations because the WHM toolkit is pretty straightforward: except for Stoneskin, all WHM spells and skills are about either healing, dealing damage or increasing your healing/damage output; there's no damage mitigation, no debuffing and barely any shielding. You'll either restore HP or blast the enemy with your spells. For that reason, WHM has a healing/damaging tool to deal with any kind of situation: burst? Cure, Cure II, any spell in the Stone family and Fluid Aura; AoE burst? Medica, Cure III, Holy and Assize; healing/damaging over time? Regen, Aero and Aero II; AoE healing/damaging over time? Medica II, Asylum and Aero III; emergency healing? Benediction and Tetragrammaton; emergency AoE healing? Assize; negative status effects? Esuna; someone died? Raise; need faster casts? Presence of Mind; need more healing potency? Divine Seal. There's no set rotation for healing and you must deal with every situation as they come. Some pointers:

- Cure or Cure II? Cure is more MP efficient and you should use it as your main healing spell. Cure II costs more, but allows you to save time and it must be used whenever you know that you won't be able to afford the time to cast Cure two times or when you know that your Regen ticks won't be enough to keep your party members safe from harm. Also, using Cure often raises your chance to trigger the Freecure effect, which allows you to cast Cure II for free;

- Cure III, Medica or Medica II? Medica II is your most potent healing spell, but it takes 30 seconds for it to reach its full potential (700 combined potency); Medica is your weakest AoE healing spell (300 potency), but it has a nice range; Cure III is your strongest AoE burst (550 potency), but it is harder to use: it affects your target and everyone close to it, which means that it will only be a good option to heal the tank and melee that are close by, or you'll have to ask your party to stack;

- Divine Seal: it has a short cooldown and can help a lot, even with MP management. Your Cure spell will become a lot stronger and your Medica II will reach over 900 potency. Learning the fight is very important to use this effectively, though.