Before I started multiboxing I had a friend who boosted his toons (non multiboxing just multiple accounts) with low toons at the door, with druid gathering up the whole instance and AOE within XP range. worked quite well in fact. Druid bear tank of course. Seemed to be the class that at the time took less damage when multiple level 60 elites start pounding on you.

But ret pally may do more damage. Then again feral cat may do just as well.

When I did start multiboxing with two other accounts, I boosted with my hunter. Quite effective IMO, however took more damage cause being pure DPS and not having any avoidance or DR from plate armor.