There wasn't really a big warlock build because each tree was viable for PVP.
Soul Link was popular for 1 on 1 encounters, survivability, and an absurd advantage against caster classes with the felhunter.
Destruction was popular because of bane, ruin, shadowburn, and conflag. It was the way to do damage quick as a lock.
Affliction was great against unorganized teams. Just run around and cast curse of agony, corruption, and sometimes siphon life on everything.
When I PVP'd I mostly played AV and my favorite build in there was 9/21/21. I'd demo sac a VW so I could lifetap constantly and recover from encounters quickly, have instant cast corruption, improved lifetap for more mana returns, and be able to stop and hit quick and hard with bane, ruin, and shadowburn.