Here is a quick list of the best enchants for not only Deathknights but all tanks in Cataclysm. Organised by item slot. A great site to see how your doing on gear score and enchants is
WoW-Heroes.
Head
Shoulder
Chest
You have two choices here. I am personally inclined to saying that
Peerless Stats is the better of the two is you take all tanking aspects into consideration such ans survival, threat and damage. Of course if all your going for is a huge health pool then
Greater Stamina would be for you.
Pants
Boots
Three choices for this slot. With the new patch 4.0.6 lurking around the corner the
Mastery enchant might not be the best choice for Deathknights due to the mastery nerf we are getting. Of course if your tank class relies heavily on mastery and you already have a speed boost go with the
Mastery enchant. If you rely heavily on master and do not already have a speed boost I would go with the
Lavawalker enchant. And if you are laking in stamina and/or need a speed boost go with the
Earthen Vitality enchant.
Bracers
Gloves
Two choices for this slot. Again with patch 4.0.6 and the DK mastery nerf coming I think DK and other tank the do not rely heavily on mastery or are laking in HP