Discipline priest is by far the easiest. Holy is also good, harder tho, but raid leaders avoid picking it in my experience ( it's not bad, but less preferable).
To answer the question, discipline is the easiest, you ll find it by yourself by the time you try it. ( However, i find it quite challenging healing alone in 10 person raids, but more likely is me being unexperienced.
Discipline priest is by far the easiest. Holy is also good, harder tho, but raid leaders avoid picking it in my experience ( it's not bad, but less preferable).
To answer the question, discipline is the easiest, you ll find it by yourself by the time you try it. ( However, i find it quite challenging healing alone in 10 person raids, but more likely is me being unexperienced.
There are no hardest or easiest healing specs. Each one is simple and difficult in it's own way.
Saying that one is harder or easier than other just comes down to personal bias.
I would say as far as stress goes from most to least is Holy Pala > Resto Sham > Disc Priest > Resto Druid.
I say that based on mana management, gearing, a raid's dependence on your abilities, and the actual healing. Holy Paladins for example are expected to maintain buffs, they have very critical life saving abilities that require good timing, mana management is highly critical, and they are tank healers so a lot of pressure. A lot more stressful in my opinion. Resto Druids on the other hand have low gear competition, easy mana management, raids will ask for combat rez or innervate but they tell you when they want it and you have time, barely have to dispel ever, and the healing is super chill. In many cases for large raids resto druids aren't even needed, you just sit there and blanket the raid or fill in gaps.
All healer specs are easy to play, but if you make mistakes then whole raid will feel it no matter what class/spec you have choosen. Healing will get harder if you aim to get BiS gear, since you will have to keep track of everything that is hapening to raid durring fights.
I would say that you go as Resto shaman - easy to learn, rich toolkit, you share emblems with warriors and hunter so you will get those pretty fast. Only you will have to get engineering to fix mobility issues class has (but engineering is way to good to skip for any class/spec). And if you enjoy playing as one then shaman offers pretty high skill cap to master.
Biggest downside to shamans is totems and interface to use them. Most fights you just put them down and forget about them, but in the ones where there is movement you now have to keep track of their placement. Also you have to keep track of your mana, even at BiS gear you can oom yourself if you are not carefull with how you use your spells.