Hunters are the worst choice, followed by mage/warlock (because they get healing spells, and only feral get something good from warlocks). The other are neither bad nor good, but don't bring any real benefit to the raid.
This is what i think are currently the choices to go, because they provide good spells for both parties, and have impact on raid performance.
Balance/Any Warriorspec (this is probably the best combination)
Resto/Prot or Retri (The only tank CD for resto, and a very good DPS CD for Rets or a nice Tank CD for Prots)
Feral/BloodDk (Damage CD and TankCD)