The Chaplain

The Chaplain is often overlooked as an HQ choice but has a lot to offer - generally speaking:

- Re roll hits the first round of close combat

- Grants fearless to the squad to which he's attached

These are both very useful in the current meta. Re rolling hits is really good versus invisible enemy units and not having to worry about breaking off the table is very powerful for a beefed up command squad or whatever unit he joins. I think Chaplains are often overlooked in favor of Librarians. You can have both though and the addition of automatic abilities should not be under estimated.

There are a lot of flavors for this HQ besides Space Marines which grant some other similar abilities that are just as good if not better:

Space Wolves Wolf Priest
Each turn you can select which type of enemy unit he grants preferred enemy which overall can be more useful if properly applied. Join the Wolf Priest to a Space Marine command squad and you'll soon see how good is re rolling 1s to both hit and wound. This is a huge buff that'll quickly pay dividends. Honestly I think preferred enemy is one of the best of the universal special rules. Of course the Wolf Priest also confers fearless as well. The Wolf Priest also confers a 6++ feel no pain which while not great is still better than no feel no pain. If you've got some points left over you can arm him with a nice Space Wolf relic like Black Death.

Dark Angels Interrogator Chaplain
This Chaplain is initiative 5 and has three wounds which is a direct throw back to older editions. I typically field this HQ over Sammael for my Ravenwing army. He's a bit cheaper even with a power fist and the Unseen Eye (i.e., Dark Angels relic) which confers preferred enemy versus all enemy units which is obviously the best.

The Space Marine Chaplain is a good choice for White Scars since you can kit him the Hunter's Eye relic which grants ignore cover. Anything that's automatic is better than a randomly acquired psychic power you have to roll in hopes of getting.