I'll comment on two of your options as healer:

Priest - My priest is one of my favorite characters because she's just such a powerful multi-boxing healer. Her mana regen is off the charts, her single target heals are massive, but AOE healing is where she shines. Most of the time I'm not even worried about her mana and if someone gets damaged i'll just use Prayer of healing and not even bother with micro managing healing. Of course I don't spam Prayer of Healing to do MT healing, but for any splash AOE dmg she's so easy to use compared to a paladin or shaman.

Paladin - Now I run heroics now with a paladin tank and paladin healer (gearing my healing pally up with tanking gear). I LOVE flash of light spam. I'll just bind flash of light to my 'open fire' button and when i start combat she'll just keep healing my MT. If he gets low I'll use her bigger Holy Light heal or just throw a few quick heals out with my shamans (or in your case whichever 'backup' healer class you'll use). I can't tell you how nice it is to do this. Her mana usage is on par with my DPS, so she's not going out of mana, she's more or less my fire and forget FoL spam MT healer. For AOE spalsh dmg i'll whip my mouse over to that screen and 'mouseover' heal the guys that need it. Pain in the ass, but it's just one of the downfalls of a paladin healer.


Shaman - Haven't spec'd a shaman to resto for multiboxing so I won't comment.

Druid - Never played one past 20.