First of all it depends of your talent tree. Second if you ask me Moonkin is the best but as i said it depend from your build. I mean if all your talents are in balance you would definitely have problems in pvp(except if your aren`t very* skillful whith the druid)and mostly you`ll be doomnik in this case :). And for those who are afraid to become OoMkin well you shoul get some gear which restore mana + the talents: dreamstate,intensity...But if you want to be successful in pvp i think that the best build is :

Required Talent Points: 61

Balance Talents -  46 point(s)

# Starlight Wrath - rank 5/5
# Nature's Grasp - rank 1/1
# Improved Nature's Grasp - rank 2/4
# Focused Starlight - rank 2/2
# Improved Moonfire - rank 2/2
# Insect Swarm - rank 1/1
# Nature's Reach - rank 2/2
# Vengeance - rank 5/5
# Celestial Focus - rank 3/3
# Lunar Guidance - rank 3/3
# Nature's Grace - rank 1/1
# Moonglow - rank 3/3
# Moonfury - rank 5/5
# Balance of Power - rank 2/2
# Dreamstate - rank 2/3
# Moonkin Form - rank 1/1
# Wrath of Cenarius - rank 5/5
# Force of Nature - rank 1/1

Feral Combat Talents -  11 point(s)

# Ferocity - rank 5/5
# Brutal Impact - rank 2/2
# Thick Hide - rank 3/3
# Feral Charge - rank 1/1

Restoration Talents -  4 point(s)

# Furor - rank 4/5


It`s not the easiest build but i think it give`s a lot of opportunities, and that makes it useful in pvp ;)