I haven't kept up with super meta "competitive" (I use that term VERY loosely) PvP since Burning Crusade, but if I had to make an educated guess- it's probably the Frost Mage. The Ret has his bubble and can break a slow, but any Frost Mage should be able to kite them well enough to either nuke or widdle them down.
That said, like I said, I have no fucking clue what state either class is in right now or even what their talent 'trees' (I use that term even more VERY loosely) are, so I have no real idea either way.