You should consider the roles of the tanks at more end-game, too. DKs have always been proactive tanks, like warriors, abusing cooldowns before you take big dmg, and making sure you avoid as much dmg as possible. (fort / reflect / etc) whereas a druid and a pally are a little more reactive (take dmg, and then use cooldown, argent defender USED to be auto-cast etc)

I think I would go for a pally or druid still, because of the great toosl both bring to the table, and a DK just before a warrior as last place. Druids and pallies also have 3 roles to play, where a DK can only really do 2 (with similar gear, mind you) and a warrior... well.