In pugs, or pre-made? With which affixes? Because the list will be quite different based on that.
In general, holy priest can pump huge heals, and is very easy to play. All you need to do is maintain your legendary (but omg, doing that is annoying). It’s an easy, safe pick. But mighty annoying to play, and the damage output is bad. Resto shaman is also fairly easy to play, can pump some large heals, you don’t have to maintain an annoying legendary (because all your good M+ legendaries are DPS leggos), and does nice damage, especially if Kyrian. Both of these perform well in pugs and pre-mades alike. Holy Paladin is easy to get started with, if you go Kyrian - but it’s harder to master than the other two, and Venthyr is a no-no for pugging, unless you vastly overgear the content.
When it comes to the other healers, pugs aren’t your friends then: mistweaver is melee, and that always makes healing more difficult. It’s also a healer that can pump big heals, but at the cost of huge chunks of mana. Not a great thing in pugs when you have to spot heal mistakes. Resto druid is hard to master, because you heal before damage comes in, so you need to know the patterns, and expect them. It’s kinda terrible in pugs when people take random avoidable damage 24/7. Disc priest is similar, but has even less tools than druid to top people off. Mind you, disc is pretty nice to pug with, as long as it’s not grievous week. For grievous week, you go holy.
Now, for premades, the list is different, a bit! Resto Shaman is amazing for that: still easy to play, and does good both on the healing and damage front. Holy priest is still easy to play, but falls behind shaman quite a bit, because of the legendary annoyance, and doing less damage. Holy Paladins are usually coerced into going Venthyr for premades, and playing Venthyr Holy Paladin is hard. It’s rewarding whenever Ashen is up, but otherwise its pure pain and suffering, so Venthyr Holy Paladin goes to the bottom of the list, as far as ease goes. Disc priest, however, I’d put behind shaman when it comes to premades (except on grievous weeks, when it falls down to the very bottom, and then gets buried even further), because when the party takes less random damage, disc is a whole lot easier to play, and also a whole lot more fun.
My own rankings, based on what I find easy or hard, based on my own experience (mistweaver in BfA, up to +20s; resto druid and venthyr hpal in SL S1 up to +18-19s; resto shaman in SL S2 up to +21s; disc priest in SL S2 up to +17-18s, holy priest in SL S2 up to 16-17s), from easiest to hardest in premades (where I primarily play):
Resto shaman > disc priest > holy priest > resto druid > mistweaver > hpal (venthyr).
However, this is my own experience, after having played the respective specs for quite a bit. When I started out, and was new to the respective classes, it was rather different: holy priest > mistweaver > hpal (kyrian) > shaman > resto druid > disc priest.