That's a vague question... they're all good at different things.

Assuming comparable gear, and asside from other benefits like their class specific buffs, etc.

Priests have the best big heals and the largest mana pool.  This makes them good at focused healing, particularly on the Main tank.  They're also the only class to have usable multi-target heals (prayer of healing, circle of healing, prayer of mending), which makes them good in situations where you have a lot of people taking consistent AOE damage.

Paladins have the most efficient heal in the game in Flash of Light, but because it's a small heal, they're probably better doing support healing; however, they're really good in situations where the Main tank is taking massive chunks of damage at a time, where a crit get him killed between greater heals.  Having a Priest do GH does the most healing, but Flash of Light helps counteract the variance caused by the crits.  IOW, having a paladin assist a priest on the MT is superior to having two priests.  Not to mention, Lay on Hands and BOP are the ultimate in "OH $^*&!" management.

Druids specialize in HOTs.  Also, have a Tree of Life druid in the MTs group is otherworldly.  And, not so much a healing ability, but being able to Rezz in combat is also awesome.

Shamans are probably the worst healers of the bunch, simply because they're very mana inefficient, and have small manapools.  But their Chain Healing ability is great for off-healing, and Stone Skin is great to have on the Tank.  Of course, their Self-Rezz abilitiy is a Godsend when there's no SS up, or he drops and is still needed.

IOW, they're each good in different situations.

If you want HPS, then the priest wins.  If you want mana efficiency and "OH $^*&!" management, the Paladin wins.  If you want utility, the Druid wins.  I'm not 100% sure where Resto Shamans stand, simply because I haven't played with many, and the one in our raid only respecced a couple months ago, and is lagging in gear.

Resto shammy = a god in a healers grp. Wrath of air totem= 101 +healing and mana sping which is, with talents, about 30 extra mp5. also dont forget mana tide totem which restores 6% of max mana per 2 secs for 16 secs with a 3min CD (or something like that) so with resto shammy in ur grp, its almost impossible to run out of mana

ok stupidnoob what you said about shamans being the worst healer class of the bunch, just because they dont spam casts with very low mana cost doesnt mean they dont have mana efficiency. The main attribute that all raiding resto shaman should look for is mp5 because of how much their spells actually cost. I have been raiding with a shaman for over a year now and with chain heal and the ability of mana tide is considered one of the best. Once you see the end-game fights in post BC you will understand what i am saying when shamans are one of the best healers in a raid and incredibly vital to a raids success. Not only do they bring totems, bloodlust and earth shield but also a aoe heal which can be spammed at a lower rank to increase mana efficiency. Paladins are good yes, but the only spell they should be spamming is the 1.5 second cast. Priests spamming big heals and healing touch's and druids throwing up a ton of hots, but most of these classes overheal incredibly and with a shaman most of the time you are healing the raid and melee with the new fights. But in my opinion raiding as a shaman i was top 1-4 in healing meters virtually every fight and yes i have seen more than gruuls and kara with successfully healing through SSC and TK

Hey! Druids got an AoE heal, Tranquility