Terrorbound Nexus appears to be the best by a large margin right now.
So far my Kara Chest + Deteriorated Construct Core + Terrorbound Nexus have been the best combo so far with real world +3 target tests and simming.
Corrupted Starlight is also pretty good.
Been simming all morning.
Aran's is now mid-tier trinket.
Here is my results so far off gear I have with +3 target.
1. Terrorbound Nexus 5.2% Total Damage
2. Kara Chest/Deteriorated Construct core Combo 4.4% Total Damage
3. Corrupted Starlight 4.0% Total Damage
4. Windscar Whetstone 3.4% Total Damage
5. Deteriorated Construct Core(no chest combo) 3.3% Total Damage
6. Kara Chest/Aran's Relaxing Ruby 3.3% Total Damage
I don't have an unstable horrorslime to test.
Also of note, the neck Choker of Barbed Reins with Mark of the Hidden Satyr enchant = 4.9% total damage. It also appears to scale more damage with more targets.
With the neck and good trinket combo, you can get almost 15% extra damage with +3 targets.
- - - Updated - - -
Here are my results off gear I have with +5 target.
1. Terrorbound Nexus 5.6% Total Damage
2. Kara Chest/Deteriorated Construct core Combo 4.8% Total Damage
3. Corrupted Starlight 4.4% Total Damage
4. Windscar Whetstone 3.7% Total Damage
5. Deteriorated Construct Core(no chest combo) 3.7% Total Damage
6. Kara Chest/Aran's Relaxing Ruby 3.7% Total Damage