Any really are noob friendly if you read the spells and use them logically ( except disc I guess but meh ) Any healer will also be viable for heroic/mythic progg and the best ilvl drops from m+, so basing this only by which I think are easiest to learn and master.
Holy Priests are super basic and straightforward. The hardest part is making the most out of holy words, and even then. Generally the easiest to play, learn and master. Imo the hardest and most annoying part was learning how to not get chunked early Legion/BfA expacs by lower lvl m+ mechanics other healers could survive. Not much in the toolkit to help you cast on the move though, unless a holy word is up. They also had lower dmg than other healers ( less of an issue for casual players )
Restoration shaman are also pretty easy to get the hang of. You might need to get the feel for chain heals range and making sure you heal the correct target with it so it links properly. In raids people will be stacked so youll have no issues there. Spirit link totem can take a bit to get used to as a cooldown and placing it. Cloudburst totem allows for more complicated gameplay. ( less so now a days but it’s still decent ) Lots of utility and spirit link make shaman valuable in early raids. Has an interrupt and aoe stun.
Paladins are pretty basic too, the hardest thing youll be doing is making sure you use the utility and cooldowns properly and the wonky mastery requires you to be aware of your distance when healing targets, not sure if long arm of the law a thing still? cause I hated that and HPaladins mastery. Always ends up one of the top 2 in m+ due to great utility, dmg and ST/tank healing.
Mistweaver has a lot of interesting stuff going on. You need to maximize ren mists for vivify combos, mix in Essence font to maximize mastery on the vivify cleaves Etc, the mix of HoT and reactive healing can be a bit tricky, soothing mist being channeled and allowing for instant cast env mists takes time get to get used to. A very fun class to play and weave damage in and there are healing talents built around doing that. They also do have great tank healing and some defensive cooldowns. Ive rolled off a few raid boss platforms in my early monk days, be careful
Very easy to blow all your mana If you dont pay attention, but in the flipside you can do a lot of healing if needed. The talent to regen mana while doing dmg is fun.
Druid imo took me the most getting used to because it is the only heal over time healer and thus the more you know fights and timers the better you will be and easier time youll have. I personally just dont like HoTs and it can be more frustrating healing people back up - mostly because I usually have to pug instead of run with people who are reliably good at avoiding bad stuff.
The spells are straightforward but dealing with burst damage can take some getting used to if you are used to reactive healers. It’s an easy class to play but harder to master and use every ability imo. I imagine in a group of friends of guildies who dont stand in bad Its less stressful. This is a strong class in m+ because of a lot of reasons, has a tank cooldown and B rez, the HoTs allow you to shift to cat form to dps too. It takes longer to master, this is why I personally put it lower. A newer player might have an easier time learning druid than say someone like me whos always ran reactive healers and cant trust pugs/hot’s imo.
Disc is the hardest for obvious reasons, dealing with atonement, does better in a friend or guild premade/non-pug group, etc.
I honestly miss cata disc I’ve tried multiple times to find something I like about the new/reworked disc in legion/bfa but I cant so Ive given up on disc myself.
Anyways that is my opinion on which healers are easiest to learn and master. ( in that order )
Now a days they’re all pretty easy to learn, mastering them takes a bit longer for some, and you can be viable for max lvl content as any healer, if you are looking to be meta/the best and to do the rated/top tier stuff youre options seem to be paly or druid, theyve been at the top sans nerfs for the last 2 expacs and given their toolkits theyll still be good at pushing extremely high m+ stuff.
Id suggest picking which you find is fun though, and testing them after the prepatch hits as theyll play VERY differently when the borrowed systems go bye bye soon.