Awww, i envy you

You don't have to stick to one race if you want a best setup. Use pros list as a tooltip:
warrior - mortal strike, high damage, dd\tank
priest - stamina, dispells, aoe fear, mana drain, heals\dd
pala - auras, blessings, dispells, heals\dd\tank
druid - buffs, roots, innervate, battle res, heals\dd\tank
mage - sheep, aoe, water, portals
warlock - cc, dots, soulstone, summon
rogue - lots of cc, high damage
hunter - vipersting, cc trap, +ap aura
There is a lot of different setups you can compile, and use max cross-buffs, like melee or caster group, but in my opinion
warrior+priest+pala will be the best core because it's an ultimate killing machine. Lots of buffs and dispells (almost impossible to cc such group), warrior will annihilate everyone with nonstop dispells. But this core is lacking of cc, so, 2 slots left,
mage will provide you cc you needed, water+portals(very important on leveling)+aoe.
So, solid core is
warrior+priest+pala+mage+??? You can fill the one slot with a class of your choice. Just use a pros list i gave you before. Rogue might be a good choice since its a lot of cc and good damage +benefit from warrior shout. Or warlock, so you will have 2 aoe classes in group, and if you make your paladin a tank (and warrior will be the arms for mortal strike), you can easily aoe every 5 ppl instance.
Well as i said before, there is a lot of choices, but the best core in my opinion is
war+priest+pala+mage.
Gl.