WoW- Shadow Priest, Warlock, or Mage?

So I've been playing WoW for a few months now on my Pally. I like my Pally, but I decided it was time to start and alt. My Pally is fun so I don't intend on a new melee class, I want a caster. So now I ask: Spriest, Lock, or Mage? Which is more fun? Which does the most damage? Which is easiest to play? And which is best for PVP? THANKS!