I would definitely say one of each.

Moonkin has probably the biggest passive buff in the game 3% haste 5%crit 12% spell damage.
Ele Sham will bring the biggest active buff in the game with Heroism + some nice spellpower and utility totems.
Spriest will bring your 3% +hit (yes you can get it with moonkin but it is a wasted cast for the druid and completely passive on the priest and they don't stack.) plus you get the amazing mana regen of the Spriest and some other great buffs like fort, shadow protection, fear ward (semi-redundant with shammy around)
Mage, while not an option you gave, would bring your highest DPS + int buff and Slow is always nice for certain situations. + food/water portals and the always nice PoM+Nuke, AoE slows and roots and the best CC of the available casters.

My vote of your available choices is Boomkin + Spriest + Ele Sham
Heroism, 3% hit, 5% crit, 3% haste, 12% spell damage, every class has there own cleanses and dispels, you've got yourself a serious mobile death squad there.

Not to mention you could always switch that group to holy trinity with options. Best of all though is they all have different armor proficiencies so you can wear any caster gear that drops and don't have to hunt the same item X3

Edit: You didn't give mage option oops.