I have PVP'd as a Elemental Shaman mostly, with a off-spec of Resto, for when I get bored or frustrated.
Ele offers pretty decent burst damage, however, as with any caster, I feel vulnerable to Rogues/Warriors/Feral Druids. Totems can help with melee classes closing on you, Earthbind can often be a life saver, giving you a few seconds to get some distance, and heal.
I've seen many Shaman go with Enhance for PvP. I have never tried it, personally. I do understand that it, too, offers some decent burst.
Probably the most used spells in PvP, for me, are Purge and Wind Shear. Nothing like purging a pallies buffs, or Mages, or Priest bubbles. Wind Shear is also a very decent spell if timed correctly, it interrupts a caster, closing that school for a few seconds, which, in PvP, is an eternity.
Beyond that, it's a matter of personal preference. I like Elemental because I can keep some distance, and Chain Lightning can be devastating if used properly. I am also most familiar with Elemental, so it comes naturally.