Tank - SK works. You get the group tap and Soul Flay procs to feed mana back to your casters. Warrior has Imperator's Command AA (1 min buff/3 min cd) for 40% cast speed increase and 500 mana/tick. Pally's don't get any mana regen that I know of, but they do get a 15% aggro reduc buff they can put on party members.
Healer - Druid is the best caster healer, but it's nearly impossible to main heal top content with just a druid. They don't get HoTs except an AA and their fast cast heals/group heals are all on longer timers that are locked between levels (ie you can't load like the 101/106/111 versions as they share the same timer.) Druid's bring crit bonuses, group mana savings & regen (
great wolf/skin), aura for +dd to nukes, mob debuffs/+% dmg for fire/cold.
Druid is also a pretty strong nuker and very strong dotter. In a backup healer role, they can put out good DPS. Kind of like mage, they tend to take the longest to regen if they run super low on mana though.
Shammy is good for pet classes (melee buffs) but doesn't bring much for casters themselves. Shammy never run out of mana and can do strong DPS output themseves on longer fights where they can get their dots rolling. Idol of Malos is a nice instant debuff trap for caster group.
I'd actually just go cleric as main healer. Gives you the best heals in one package. If you want a backup healer, add a druid.
CC - Ench is better for casters. More aa/buffs that add straight nuke crit %. Bards can bring the +45% to nuke dmg to the group, but their other nuke songs just add minor +dd's (post-crit calculated) to certain types of nuke. Bard actually can be a better mana regen system, though you'd have to customize your macro to swap song sets for downtime.
Casters - So you get some nice "Synergy" buffs at EoK+. Mage/Wiz/Ench can proc group buffs that adds 40-65K to each toons next nuke of a certain type. If you run 3 of a certain class, you can also use the Alliance/Covenent big procs on named. 3 Wizards is popular for that though it's not as useful for just grinding. Mobs tend to be dead before its worth setting it up.
I've run War-Dru-Ench-Mag-Wiz-Wiz for awhile and it's a nice group. The druid really holds it back though as he eventually winds up having to spam his slower cast heals and can get behind. Between the ench nuke buffs and wiz AA, they can melt a named with no issues though.
If I was remaking it, I'd probably go Tank-Clr-Ench-Mag-Wiz-Dru or skip the backup healer for another wiz/mage.
I personally love mages and used to run a pettank group with mage tank. That works really well until you have an event/area with un-mezzable adds. Like giants.
My dream caster group is Mage
(EarthTank)-Sham-Ench-Mage
(Water)-Mage
(Water)-Dru. Put all 5 pets on taunt, three mages for synergy, plus a ton of pets/swarms for mage's raid nuke, shammy runs their HoTs to heal up whichever pets have taken dmg from taunt, druid back up heals group only to focus on dps, everybody nukes including the shammy (to produce more free doggie pets!) Maybe a Bst in place of Dru might be better, but I haven't got around to leveling more mages to try this out.