Just by saying Multi-box you open up the possibility to a plethora of choices; stick to the “holy trinity” philosophy and you cannot go wrong.
For me, multi-boxing has been more about the right group synergy along with simplification of macro’s then about what is “the” right choice for a given role.

Tank – The Paladin for me is the only choice; simple rotation that is fire and forget. Once engaged I never worry about threat.

Healer – I don’t think there is a “wrong” choice here; macro setup is more complex if you don’t have a group type heal.

DPS – Any class will do – melee/caster mix make movement much more challenging however. I like to keep my healer and (3) ranged dps toons close together using the number keys for movement while the tank uses the WASD.

For me – I choose the non-traditional route going with a 5-man mixed group; currently RaFing a new Warlock to replace my Mage; nothing wrong with the Mage, I just wanted another form of wipe protection that the Warlock brings over the Mage.
Water is cheep and teleportation hubs are everywhere now.

So long as you choose 5-classes you like to play you cant really go wrong; some combinations will be MUCH harder to master then others.