Technically all three specs are viable. Frost will bring a much needed +20% melee haste (this is given my encha shamans as well, but enha shamans are almost non-existent) to the raid. Unholy will bring +30% more damage with diseases and +13% more damage with all spells against a target (this is given by boomkins as well). Blood will bring +10% attack power which is already given by hunters and I think one more class, which I can't think of right now. Anyhow as all three are viable I'll give you a list of what negatively and positively effects each spec:
Blood is negatively effected by:
Blood gorged reducing boss armor instead of ignoring it. This is a pretty big nerf in itself for blood as it forces Blood dps dks to gem for arp until 1400 rating, whereas otherwise they'd have to get only 1260.
Blood worms barely spawn and when they do, they don't attack.
Death rune mastery has a strange way of converting runes and when I played blood I could not find a good way to deal with the strange convertions.
Unholy:
Scourge strike does not double-dip but should. It does both physical and shadow damage, but only one of the two gets damage increasing effects.
The permanent ghoul doesn't receive expertise from the hit rating its master has, nor does it take less damage from incoming boss/mob/player AoE.
Both ghoul and Gargoyle are claimed to deal low damage. This I cannot confirm.
Frost has no negative effects, at least to my knowledge, but it has a possitive one:
Frost strike receives damage modifiers twice (it double-dips) causing it to do way more that it should.
Note: 2pT10 is claimed to be broken for all 3 specs. In reality it is not broken for frost, but I can't say if it is or is not broken for the other two as I haven't done any tests on those.
With that said, when it comes to raiding you will more often than not get a raid spot as frost rather than unholy/blood. I hope I helped you with your choice.