IMO, Death Knight tanking is pretty fun. Also, the Death Knight is doing quite a bit of damage while tanking, which overall helps out a lot. As per my signature, I've got the Shaman and Mage for the obvious benefits and the Moonkin since I run an all-caster (minus the DK Tank) group. Also consider what you want for backup heals and things like that. Because I've got a group with two hybrids, off-heals and things like that come quite easily.