stuck with same prob except for all my tanks are lvl 1. So the thing to really decide is the time between leveling (like the 70 pali or the 62/45), does not compare whatsoever to the times you would spend with them at 80. So go with the tank that you want not what level they are.

But, as a decision, palies are the easiest to box with but or kinda boring from what ive heard (massing tank button)
Dks are harder, but provide a dps bonus to shammies
Warrior, ive never seen a multiboxer with warrior.. dunno ;0