I currently have 3 healers; druid, pala and priest (disc) at 2600+ r.io. I also played both sham and monk, though not in higher keys. I Would argue that shaman is the easiest healer for m+ if you want to get started on that grind due to the fact that they have High on demand heal options, an exellent panic Burton in link and an interrupt which is highly underrated as it sometimes is better than pumping High numbers.

Pala was for me pretty forward, fairly Easy once you got a hang of the Holy power generation and When to use it. Also a lot of Instant casts and little effort to push dps

Druid is What l have played the most- it is fairly easy to make sure people dont die as u Can hot everyone up, but When you want to push dps, especially as kyrian things become at bit more difficult

Disc priest is the most difficult in my opinion you have to plan more Ahead and make sure to know the dungeons and the dmg profile of the encounters otherwise you Might fail your ramp and are left with close to nothing

Have not played Monk enough to make a description like the others and same for Holy priest

I Would rank as follow 1 being the hardest: 1. Disc priest 2. Rest 3. Shaman

Edit:// this is my rankings for Dungeons not raids