healing is my jam
paladin heal: probably the worst, because it pretty much has to longcast (as opposed to instant cast) everything and it's aoe heals are terrible, meaning you're only really good at healing when only the tank takes damage, which, while optimal, is really never the case. what you can do other classes can do WHILE healing the rest of the party. you will be stuck being 1/many heals in a raid scenario, parked on tank, or not invited at all (everyone already rolls pally tank and ret O_O) really a no go overall, in my opinion.
shaman hea:l seems good at first but really excels in the 5 man max and pvp type scenario. having to long cast almost everything is a drag anytime you're really needing to heal alot is, it's really not good in big groups. chain lightning only heals within the party it's cast at, really doesn't work in ten man or greater. shoutout to the era when it made sense. shamans are excellent for group composition overall due to the blinding excellence of totems, excellent passive raid buffs from talents and heroism but you can do that while you dps, and they really stand out there compared to healing. shamans are dope in pvp cuz they're tuff as nails with great tools. stack dat haste, tho.
dic priest: excellent for tank healing, just barely acceptable for group healing, amazing for pvp. bubbles work for tanks and party mates in pvp, not really the best for trying to keep a whole squad up, acceptable for dungeons usually, just need the right spec and glyph for it. fun, effective and easy to gear.
holy priest: is pretty terrible at everything except picking up the slack where the tank-only healers lack; healing the rest of the party. even still they need so much haste - due to all relevant spells being longcast AND especially long to cast even specced for it - i wouldn't recommend trying to play this spec unless you're very close to BIS with an emphasis on haste according to your level, or else everything just going to die on your watch, either cuz you're too slow or cuz you're ****ing OOM. this tree does no favours.
druid: healing perfection. what it lacks in tools like shaman's amazing totems, priest control and dispells, and paladin's toughness it makes up for with one thing: being the best damn HEALER in the game. you barely need to hardcast anything. on my favourite raid/dungeon healing spec i only cast nourish, the rest is just instant hots and other instant cast spells. which means you have the ability to move around, can't be interrupted or "pushed back" easily and react quicker without being stuck in spellcasting. also if you spec correctly you come out very tough, with protection close to what a shaman's is with a shield. there's really nothing wrong with this spec, you basically have infinte mana especially compared to other healing classes.
Another excellent thing is that druids are the only class in the game to do 4/5 possible roles that any character could potentially play; ranged magic, melee dps, tank and heal. the only thing they're lacking is ranged physical, which only hunter does anyways. this lets you play ONE character, and as long as you're keeping gear around for it, be a part of any raid in any needed role. not to mention that they're healing, tanking and dps (when properly geared) is arguably some of the, if not THE best choices in the game. you won't need to reroll a new character if you want to play another role, just hit up the trainer and reset skills and buy new glyphs. congratulations: you've saved yourself hundreds of hours.
and, for icing on the cake: as healers you have a specific niche in gear that absolutely zero other classes utilize: spirit-casting-leather, which makes it extremely easy to gear compared to shaman and priest.
go druid, you won't regret it.