Gul'dan Set (Tier 9 PTR)
(2) Set: Increases your pet's critical strike chance with its abilities by 10%
Conqueror's Deathbringer Garb Set (Tier 8 Live)
(2) Set: Increases the damage done by your Unstable Affliction by 20% and your Immolate by 10%.
Personally, I prefer Tier 8 (2) Set Bonus, as it gives definite damage increase rather than "pet" critical strike chance. However, Destruction Warlock with Empowered Imp and Demonology Warlock with Demonic Pact will benefit most from the set bonus.
Which one you prefer?