I'd suggest 1x DK tank and 4x shaman. The DK tank gives all your shaman 13% more damage, since he brings ebon plague debuff to the group. I have a 80 pally tank, 80 druid tank and 80 DK tank. Even though my pally is better geared, the DK tank is by far my favorite. The druid tank sucks since he has no good AOE damage.