What is the best race of Warlock?

Short answer: For PvP, Gnome, Orc, or Undead. For PvE, Human, Orc, or Undead.

Long answer: First of all I will state that the stat differences between the races are pretty much negligible at level 70. Thus the factors in choosing your race for your Warlock are in its appearance and the racial abilities. I also want to state that it is probably a bad idea to base your choice of race by a specific talent spec, as you WILL respec eventually and the difference isn't really THAT big.

For more information regarding racial skills, check the official website.

The best two races for PvE are Humans and Orcs - one being good as a time saver and the other having higher potential raid DPS. For PvP, it's a toss-up between Gnomes, Orcs, and Undead for their anti-CC abilities and the Orc's two DPS racials.