1)Druid you want them since they top the healing parse (and ubba AoE heals) WRONG
2)Hpal since they make the best Mtank healers DEBATABLE
3)Rshm get the best heals when your teamate are low on HP. ONLY IF THEY STACK MASTERY
4)Priest ? VERY VERSATILE
But what is the power of a priest ? Is it just because they can shield your tank ? or to replace the Rshm in heavy mobility fight ?
1) druids have great aoe heals while running around. their strength is their ability to MOVE and heal. sure, theyre terrible at healing one person, but holydins are the best aoe healers atm.
2) holydins are great MT healers, yes, but theyre the BEST aoe heals, assuming you have a good group that knows how to stack up when needed.
3) shams are only great when people have low health IF they stack mastery. on progression content most shams DO stack mastery, but haste is also great for them. they are also very good when people are stacked up.
4) priests can fill a variety of roles. disc has awesome tank healing, which is ON PAR with holydins (imo). holy has great AOE healing which is on par with shams (imo). lightwell, lifegrip, divine hymn, and mana hymn (?), are all great assets of priests. their PoH is sometimes a curse and sometimes a blessing. healer A on group 1, healer B on group 2, etc... priests excel on these specific encounters. also... priests just have SO MANY BUTTONS that they can be brought to damn near any raid and be useful. assuming theyre a good player.
Imho Paladin is a rather weak Tank healer atm on a lvl close to druids. But they are an awesome "secondary" tank heal thanks to beacon and stuff also the strongest Group heal when stacked but weak when moving a lot.
disc priest can do awesome tank heal when specced/geared for it or mitigate spikes in high AoE situations with PoH (and heal for quite a lot) both while never really running oom.
Druid is rather weak with tank heal but still can do it if necesary but has very strong Group heal and 1 nice and one very strong group heal cooldown with ToL and Tranq. Also never really running low on mana and probably the best healer while moving.
Shaman great tank heal (on par with Disc imho) and great groupheal when stacked up. Weaker then the rest while people are not stacked.
and last Holy priests... I Dont know if I just never met a good holy priest or what but personally i would always prefer A disc over a holy. decent groupheal decent tank heal. A great Tank CD but rather weak Grp heal cd (hymn is still strong but I prefer every other CD out there)
So Paired with A paladin Id take A Disc, Shaman or Druid but Holy should be ok, too. In other words everything is fine.