Here's my personal tierlist in a 2v2 scenario:
T1:
uh dk, feral druid, ww+mw monk, frost mage, enh shaman, disc+holy priest, rogue
T2:
frost dk, blood dk, balance druid, bm monk, fire mage, ele+resto shaman, shadow priest, hunter, ret, warrior
T3:
resto druid, warlock, arcane mage, holy+prot pala
The classes are not listed in any particular order within the tiers. Also if I didn't bother to write each individual spec for a class, it doesn't mean that all 3 specs are equal, but rather that all 3 specs belong in the same tier.
edit: actually I'm just gonna do a big ranklist for accuracy. Keep in mind this is my opinion based on a 2v2 scenario. Not 1v1. Not 10v10. 2v2.
Feral Druid > Discipline/Holy Priest > Unholy DK > Subtlety/Assassination Rogue > Windwalker Monk > Enhancement Shaman > Combat Rogue > Frost Mage > Mistweaver Monk > Balance Druid > Elemental Shaman > Marksmanship/Survival Hunter > Retribution Paladin > Protection/Arms Warrior > Shadow Priest > Restoration Shaman > Fury Warrior > Frost DK > Fire Mage > Brewmaster Monk > Blood DK > Guardian Druid > Affliction/Demonology Warlock > Holy Paladin > Resto Druid > Arcane Mage > Destruction Warlock > Beast Mastery Hunter > Protection Paladin