blizzard sorc. the reason i say cold is cuz cold mastery lowers resistance, which hurts more than the other masteries that just increase the spell dmg.
ie. lets say that my blizz does 4.8k dmg. cold mastery drops u down to -100 resist. pvp penalty = 1/6, -100res = 2x dmg. total dmg dealt to u = 1.6k. my 1.8k ice blast would be dealing 600dmg. beware of ppl who stack resists.
lets say my fireball does 12k dmg. after pvp penalty i end up with 2k dmg b4 including resists. so if u has 75res, im dealing only 500dmg. so bigger numbers arent always better.
lightning sorcs...i guess its possible to make a decent one, as i have seen one that did surprisingly well. due to the fact that lightning is the most resisted and absorbed element in pvp, u probably wont win as often.