World of Warcraft Unholy Death Knight is in my opinion the best all around PVE raiding spec available. The Death Knight in all specs is able to tank or dps , Some specs are more suited for one or the other.

The Blood tree has very unique survivability traits thus makes the Blood tree handy in pvp and as a tank as well. In PVP the Blood death knight has a very high burst dps which is excellent in PVP [battle grounds, arena]. The Blood tank has many survival talents which can be speced into which makes the blood death knight a nice choice of tank.

The Frost Death Knight has a higher armor build through talents and veins of ice which make the Frost Death Knight very indestructible.When suited up in the T8 plate armor[Uldar] the focus on the Frost Death Knight changes to more of a high shadow damage out put  thus makes the Frost build very high dps.

The Unholy Death Knight is a very AOE based spec which is a high out put of dps with a few points in the blood to reduce agro threat,,As the Unholy Death Knight Tank the high out put of magical spell and magical dots creates a very high threat table which makes the Unholy Death Knight very viable for both tank and dps.