You want to have the following trinkets: Discordant Chorus, Reaper's Harvest, Unending Hunger, and Vial of Convulsive Shadows.
Generally speaking you want to use Unending Hunger + Vial of Convulsive Shadows for ST/cleave fights, and Unending Hunger + X for AoE fights (X depending on your circumstances, but mainly Reaper's Harvest or Vial, sometimes DC) .
Hellfire Assault: Unending Hunger, Discordant Chorus, and Vial of Convulsive Shadows are all nice.
Iron Reaver: Unending Hunger + Vial of Convulsive Shadows
Kormrok: Unending Hunger + Discordant Chorus (or Reaper's Harvest if you contribute most of the damage to the hands)
Kilrogg: Unending Hunger + Vial of Convulsive Shadows
Hellfire High Council: Unending Hunger + Reaper's Harvest (or Vial of Convulsive Shadows if playing Breath)
Gorefiend: Unending Hunger + Reaper's Harvest
Iskar: Unending Hunger + Reaper's Harvest/Vial
Socrethar: Unending Hunger + Vial of Convulsive Shadows
Tyrant Velhari: Unending Hunger + Vial of Convulsive Shadows
Fel Lord Zakuun: Unending Hunger + Vial of Convulsive Shadows
Xhul'horac: Unending Hunger + Reaper's Harvest/Discordant Chorus
Mannoroth: Unending Hunger + Reaper's Harvest/Vial
Archimonde: Unending Hunger/Vial + Reaper's Harvest (on mythic, for heroic Unending Hunger + Vial with BoS)
Reaper's Harvest is dispensable on all of these fights except Council if your raid has lots of burst AoE - it's only a solid gain if Wandering Plague is ~9% of your damage.