Tank death knight is a must have for 10man and 25man! You can have many variations of talents and can use even other specs, depending on your gear, content and boss fight, and of course your preferences. Blood is the strongest tanking spec in the last content with the last patch, but frost can have it's advantages on fights like...
Blood is considered as a not valid dps spec by many, but it can have it places if the itemization and the playstyle is correct. Blood works similar to fury warriors gearing wise. Blood can be a good damage dealer, but will rank lower than the other two specs, but on single target fights blood can be really good.
Unholy death knights are really good, you always want one for both 10man or 25man raids! They provide damage support and great damage and even greater aoe damage on fight like the Lich King. With Shadowmourne they shine even more.
Frost death knight is a strong single target and a good aoe dps. It can burst and switch pretty quickly and can provide a little support to the raid (+20% melee haste). It starts out with strength stacking then in higher gear it is advised to go for armor penetration since the two top of your damaging spells (single target) is melee and obliterate....
