There are a lot of Death Knight specs rolling around out there the question is which ones are the "best" and why?
Here is what I have compiled from my research and trolling around forums:
Blood Specs:
51/13/7
This spec is considered the best PvE DPS spec at the moment. You pick up talents such as Death Rune Mastery and Annihilation to make your obliterate usable for the pure purpose of getting death runes to heart strike. Heart strike is always better damage per rune, so when death runes are up use your Heart Strike. Heart Strike taking only one rune is nice, but what makes it even better is that it can proc your Sudden Doom, a key DPS talent. Most of the talents are quite obvious on why they're placed where they're placed
Variations: Some people prefer Blood Worms over Rune Tap and Imp. Blood Aura, this is a preference thought Blood Aura is more advantageous because it can conserve some of the raids mana.
Here is the rotation you should run:
IT-PS-OB-HS-HS-DC
HS-HS-OB-HS-HS-DC-DC
IT-PS-HS-HS-HS-HS-DC-OB
53/0/18
This appears to be the blood PvP spec, you get Rune Tap and Imp. Rune Tap because the amount healed is very useful and a shorter cooldown is always nice too. Vampiric Blood is used so you can heal even more, use this prior to Death Strike and Rune Tap, the combo will heal you massive amounts of health. You go into unholy because Shadow of Death is an amazing PvP talent and the reduced stun duration of On a Pale Horse is always nice to have.
Frost Specs:
13/51/7
This is the current best DK tanking spec that includes Imp. Icy Talons, this spec is only better then the latter if you lack an enhancement shaman, if you have one you have wasted 6 talent points. Most talents are self explanatory, talents like Rime and Annihilation are in for superior threat, mitagation talents such as Toughness, Unbreakable Armor, and Lichborne are included for survivability and longevity. Avoidance talents are Blade Barrier and Anticipation, Bladed Armor provides massive amounts of AP and therefore threat with Frost Presence on and good gear. Frost has very good livability and has incredible AoE tanking ability.
13/51/7
This is the alternative to the former, if you have an enhancement shaman in your raid then this spec will always be superior. You take Deathchill and Killing Machine because they are the best talents that are available to achieve the lower tiers. They are merely there to replace the 6 points lost out of Ice Talons.
17/54/0
This is the frost DPS spec. No, this is not a joke, frost does quite formidable damage when applied correctly and with proper spell priorities. This is the a great spec to choose if you need to OT for one or two fights in a raid but overall for the majority of the encounters you will be DPSing. You can tank reasonably well and you can DPS quite well too.
Unholy Specs:
I'm tired atm, I'll work on this tomorow
Please comment and make any suggestions for changes.