Affliction, demonolgy or destruction.

Destruction. Takes some getting used to, but it's high DPS and has some quick bursts for either PvP or PvE.

*Extra bit: Undead only adds cool looks to the Warlock Class... Other than that the racial bonuses aren't very useful and will just seem sort of pointless after a while.. Orc is the best for overall damage and stun reduction for the Horde side.

demo spec is boring imo plus shetty in pvp most of the time.

either go affliction for dots or destro. i like affliction during pvp because although it takes time to cast some dots they still last while im dead pluse when they run away they tick and die.

destro is always fun too its sorta like playing a mage imo. i like either.

depends on what you plan on doing.