This isn't a complete list, nor sorted by T1-T3 comps.
However, for each spec of each class, comps (for 2s) are listed from most to least viable (top to bottom).
Code:
Shaman
Enhancer
Enh/Restodruid
Enh/Disc
Resto
Resto/Ret
Resto/War
Resto/Warlock
Resto/Feral
Priest
Disc
Disc/Rogue
Disc/Hunter
Disc/Mage
Disc/War
Shadow
Shd/Rogue
Shd/Mage
Shd/Lock
Rogue
Rogue/Druid
Rogue/Disc
Rogue/Lock
Rogue/Mage
Rogue/Warrior
Warrior
War/Druid
War/Pala
War/Shaman
War/Rogue
War/Disc
Mage
Mage/Shadow
Mage/Rogue
Mage/Feral
Mage/Dreamstate
Warlock
Lock/Druid
Lock/Rogue
Lock/Priest(either)
Paladin
Holy
Paladin/War
Paladin/Warlock
Ret
Ret/Shaman
Ret/Rogue
Ret/Mage
Druid
Resto
Druid/Rogue
Druid/War
Druid/Lock
Druid/Hunter
Druid/Mage
Moonkin
Moonkin/Rogue
Moonkin/Mage
Feral
Feral/Rogue
Feral/Mage
Feral/Shaman
Feral/Disc
Hunter
Hunter/Druid
Hunter/Disc
Beastmaster/Rogue