The Ordo Hereticus are the Witch Hunters, those would would seek out and kill rogue psykers and other witches, and their Inquisitors represent this well. The Ordo Hereticus Inquisitor has plenty of wargear options to represent this in addition to options open to all Inquisitors.
The Inquisitor starts out with Frag, Krak, and Psyk-out Grenades, Carapace Armour, Bolt Pistol, and Chainsword. He is also an Independent Character with Stubborn, which is conferred to his squad. While his subpar statline (aside from 3 Wounds) would make him seem like an unattractive choice, he can take a lot of extra equipment and unlocks the Henchmen choices. Henchmen are the major part of taking an Inquisitor, but I'll cover them in a later article. Let's move onto the options.
Power Sword - With only strength 3, chances of this being a good choice are low. Even with Hammerhand, you are better off either avoiding close-combat or picking up a Power Fist/Thunder Hammer. Plus, Ordo Hereticus aren't the best close-combat option, but more on that later.
Plasma Pistol - While having the plasma shot is nice, I'm not sure if one shot (generally the best you will get a game, if even) is worth the points, even more so considering an Inferno Pistol is the same cost.
Inferno Pistol - A somewhat strong option. Melta is great, and this acts as a pistol, too! Due to low range, however, you will probably be shooting it once a game at best and thus you may be better with the longer-range, Combi-Melta.
Combi-weapon - Combi-Melta is probably your best choice here, if you choose to take one.
Power Fist - Not a horrible option. Your initiative of 4 isn't amazing, so dropping to 1 isn't blasphemous. Again, though, this Inquisitor doesn't have a lot for close-combat, so perhaps shy away from gearing to do so.
Condemnor Boltgun - While this is quite fluffy (and reminds me of a vampire-hunting crossbow gun), it just isn't very good. The wound has to be allocated to the psyker for the effect to occur, and therefore isn't all that likely. Plus, with the FAQ, the wound has to be unsaved; not all that likely for this to occur.
Thunder Hammer - A good choice for close-combat oriented Inquisitors, like the power fist. You just won't be likely building for it.
Null Rod - Another interesting, fluffy-inspired, and odd weapon. It stops you from using any Psychic Powers on yourself and prevents opponents powers, but it doesn't stop you from using Psychic Communion or letting a Librarian use a non-self targeted ability. At 25 points, though, it is a bit expensive for a situational weapon, especially since it stops you from using Hammerhand.
Servo-Skulls - You can take up to three of these for a low point cost. Good if you have a lot of Conversion Beamers or other blast templates or if you plan on attempting Deep Strikes often. They do offer scout/infiltration defense, too, so a good left-over point spending, but they don't make you more combat effective. They are cheaper here than taking them on a Grand Master.
Psyocculum - Being BS 10 is amazing, even if it is only against psykers, as some armies (Grey Knights, Eldar, and Tyranids) almost always have a psyker or many more wandering about. Stick this with 4 Psycannons and suddenly you are destroying your fellow Grey Knights. It's a bit pricy and situational, though, so again, I wouldn't really take it. It's too bad so many "cool" wargear options are either too pricy or too situational.
Power Armour - A fine upgrade, but the awkward point cost of 8 will probably mean you will have 2 points "floating" at the end of your army list.
Psychic Mastery - A fine upgrade if you are putting the Inquisitor into close-combat (for Hammerhand) or if you want to be able to control your reserves well with a reserve based army (with Psychic Communion). Note that you do not have to exchange your current weapon with a Force Sword due to Errata in the Grey Knight FAQ.
Close-Combat Oriented Ordo Hereticus Inquisitor - (75)
Power Sword, Inferno Pistol, Hammerhand
Adds a bit of a punch to close-combats with an extra Hammerhand cast, a good number of attacks with a power weapon and pistol, and a melta shot to boot.
Shooting Oriented Ordo Hereticus Inquisitor - (60)
Great to add to a squad of Purifiers with Psycannons on foot or Purgation squads and give them that extra punch against psykers.
Close-Combat Ordo Hereticus Inquisitor Two - (80)
Power Fist, Combi-Melta, Hammerhand
Very similar to the other close-combat build with a power fist instead of a power sword (and swapping the Inferno Pistol for a Combi-Melta)
Keep in mind while none of these seem particularly attractive, the main reason you take these HQs is either for a cheap HQ slot so that you can focus points elsewhere or so that you can take Henchman.
(I apologize for the copy-paste post. Blogger deleted this twice before once when I was just about done and the other half-way through. I got a bit sick of it and didn't want to keep everyone waiting even longer)
Next post will be very similar but instead covering Ordo Xeno Inquisitors.