If I had 10 accounts I'd go with the following for sure:

Paladin Tank
Bear Tank
Resto Druid
Resto Shaman
Holy Paladin
5x Mages

Mages for burst damage and versatility between fire and arcane and ease in macroing. The three different healers are not too difficult to control together - resto druid could do a castsequence completely ignoring reactive healing - just need wild growth on cooldown, refresh lifebloom on cooldown, and macro rejuv to your other two healers' direct heals. Paladins are the most powerful tank healers imo. You also cover all armor classes so nothing goes to waste, and your tanks and healers gear up fastest.

Bear tank and paladin tank gives you a powerful off-tank and dps in cat form (extremely difficult to macro but powerful if you take the time to set up the "do not advance for xx seconds" in ISBoxer) and Bears have the most powerful 4-set bonus in the game if glyphed.