HQs are vital to running a tight list and Black Templars have some serious solid choices. What’s best for you depends on what you want to do with your army. Here are some abilities I want:
+ Chapter Master Rerolls -> Makes your Eradicators armed with heavy melta rifles super killy
+ Litanies of the Devout -> We have some of the best litanies, every list should have a Chaplain
+ Utility -> heavy dreads, take a Forge Master… want a beat stick, the Emperor’s Champion, etc.
I consider High Marshal Helbrecht an auto take unit. Sure he costs some serious points but he has two weapons that are both relics for all practical purposes, and his ability to buff core units with +1S can be quite handy. One thing our HQ have going for them is they are Primaris, so don’t forget to pop Transhuman Physiology if they are caught in a tight pinch. You can roll your own but he won’t be as killy and will be comparable in points.
Grimaldus is the best choice for a Warlord versus psychic heavy armies such as Thousand Sons and Tyranids. Grimaldus takes some skill to properly utilize since he has his cenobite servitors… they must always be protected via Look Out Sir. The generic litanies are good as well so running double Chaplains can be quite effective - in particular 2" added to a unit’s charge roll can be game breaking. It’s very important that your Chaplain is very reliable when casting litanies so make sure to take the proper upgrades such as Ancient Breviary, you’ll be glad you did.
The Emperor’s Champion is a true murder machine and requires some skill to reap his benefits. The Black Sword is AP4 when attacking with the thrusting strike… this is a natural anathema to Armor of Contempt along with three flat damage. He is our most underrated HQ choice for sure.
If you like to run dreadnoughts then the Primaris Techmarine upgraded to Forge Master is an excellent choice, healing up wounds and handing out +1 to hit.
These are my top choices, let me know if I missed any you like to take.