There is no best race for a warlock.Every race that can be a warlock has its own abilities.For example
Orc:+5% pet damage,Bloodfury(+140 spell damage) and 15% Stun resistance
Undead:Will of the forsaken,+10 Shadow resistance,Underwater breathing and cannibalism
Blood elf:+5 all resistance,+10 enchanting skill,Arcane torrent,and mana tap
Human:+10% spirit,Perception,Diplomacy and sword specialization
Gnome:+5% Intelect,Escape artist,+10 arcane resistance,+10 engineering skill.

Now you tell me which race is best for warlock

I'd say that horde locks overall are better than ally, since for gnomes, they don't really need that 5% boost escape artist is alright i guess, then not many ppl but mages use arcange, and engineering isn't that usefull.

And for humans, 10% spirit is useless for locks, perception is stupid since locks get paronia(i think thats the spelling)at lvl 42 on their felhunter wich inscreases stealth for you and party members all the time, locks don't really use swords in close combat, just the stats, and diplomacy...well thats good for anyone that is too lazy to do a little questing.

I personally have a UD lock and i think they're awesome, WotF is awesome against anyone, 10resistance for shadow is alright, cannibalizm is awesome for any non-healing class, and i guess underwater breathing is only good for arathi basin as far as i know.

The orcs +140 damage is pretty good, especially for destruction locks, 15% stun resistance, its like a arms warrior, and 5% more for pets is good for pvp and soloing