New DK

So im leveling feral and on 55 im planning to create a DK (i never played it before)

I want to know about specs:

Whats best for pvp dps?
Whats best for pve dps?
Whats best for tank?

P.S. Wotlk Exp

Pvp - Unholy mostly but frost is decent, works esp on blackrocks custom patch.
Pve - Unholy dk and frost dk both have unique buffs and usually has 1 spot each. Frost dks are more common but deals more damage. Unholy dks are usually needed by guilds much more so than frost.
Tank - Blood is the dominant specc.

for PvP actually all 3 specs are quite good, but I myself prefer Unholy. Pve dps you can actually play all 3 specs aswell, Frost might be best on dps wise, unholy can be more supportive for raid and blood is good only if you are able to get 100% arp. for Tank no doubt blood is best.