I hear Night Elf is the best because of Shadowmeld. But Orc is a strong pick too because of Pet damage buff and Bloodfury. I plan to do both Arenas’ and BGs’ while leveling and main PVP at max level.
Depends on what you’re looking for. Numbers wise, I think Orc and Troll provide the strongest racial DPS benefits on the horde. For the alliance, mechagnome is insane with their stacking primary stat racial but I believe dark iron dwarf edges them out with proper use of their version of stoneform.
Orc is weaker than it was in the past, these days racials lean more into utility actives for benefits.
The throughput gains are largely irrelevant since all racials are balanced to be around 2% max margin from weakest to strongest.
This means that alliance has the best pvp racials for lock ( and most dps) by far.
Nelf is the defacto best racials cause meld has a pseudo immunity frame and all the other benefits of being able to drop combat on demand.
But for a more general approach to things :
- nelf is better into casters/ranged.
- dwarves/di better into bleeds/dots/poisons.
- velf better into standard zug melees.
These 3 races are often interchangeable as “best” for warlock depending on the meta.
There also high mountain tauren.
The new void elf buff racial is going to cause some melee to rage 
.
From what I’ve heard, Mecha Gnome works good as a Warlock, but, if you want to retain your dignity, choose Night Elf, Orc or Dark Iron Dwarf.