Shamans are the most popular DPS for many good reasons, especially with a DK tank. Although pally + 4x shaman is the most popular, due to buff stacking a DK + 4x shaman may be stronger. Although priest healers are great, you may want to consider you'll have very limited protection against fear without at least one shaman in your group. Otherwise, with a pally tank, you could consider moonkin DPS since their buffs work well with pally buffs.