You are not going to find any close combat monsters or characters able to decimate entire swathes of the enemy army in the Astra Militarum HQ section, but you will find some great characters for buffing your seemingly weak Guard units to help them perform better on the tabletop.
Lord Castellan Creed
Creed is a great buffing character and warlord to have for a Cadian Force. He has a fairly modest statline, hitting on a 3+ in combat and in shooting. He comes with two hot-shot Laspistols and a power sword, so should be able to do a bit of damage against weaker infantry. He is T3, with 4 wounds, a 4+ armour save and 5+ invulnerable save, so is not the most durable.
Where he does excel is the buffs he provides to the army. Having Creed in a battle forged army as your warlord gives you two bonus command points. Given the number of command points a Guard army can wrack up, this is a nice additional bonus to have. Creed also gets to issue 3 orders per turn and each order has a range of 12”, a nice boost over normal orders, allowing you to influence more of the battlefield. At only 70 pts, he is a really nice addition to the army and should prove very useful to include.
Probably the most popular choice in a gunline Guard army. The Company Commander has a modest statline and reasonable protection. He can take Melee or ranged weapons, so you could possible upgrade him to carry a power sword, but I would most likely keep him fairly cheap. His real bonus to me is the ability to issue two orders per turn. These are the best way to get the most of your Infantry squads, and at only 30 pts is a real bargain. He is great for boosting your army and filling out compulsory HQ slots at a really cheap price. I can see me running a couple of these in most of my Guard armies.
The Tank Commander returns in the 8th edition codex and gets better in that he no longer has to buy a “support” tank as he did in 7th edition. This helps to make him a bit cheaper to field and more flexible on the battlefield. The bonus of the Tank Commander is that you get a Leman Russ with BS3+, which should greatly increase the firepower the Leman Russ can put out.
He also gets to issue an order each turn. These either allow the Leman Russ to move and advance in the shooting phase, re-roll to hit rolls of 1 or shoot and then use its smoke launchers. I think the re-roll to hit will be of more use than the other two and will most likely be used on the Tank Commander himself to further increase his accuracy. The Tank Commander costs about 45 pts more than a standard Leman Russ, which seems like a reasonable cost. On a Punisher Russ armed with Heavy Bolter sponsons, the improved BS of the tank commander should be very useful, especially when it gets to fire twice.
Knight Commander Pask is a Tank Commander that gets to use two orders per turn and has a BS of 2+ at full strength. For only an extra 10 pts over a regular Tank Commander, this is an absolute bargain and I see no reason not to take Pask over a regular Tank Commander in a Cadian army.
Yarrick is one of the few units in the Guard army that provides a re-roll to hit bubble (1’s for most armies, full re-rolls against Orks), so that alone makes him a valuable inclusion in a Guard army. He is also fairly durable, with a boosted toughness of 4, a 4+ invulnerable save and special 3+ save against losing his last wound. His Power Claw also makes him a pretty potent force in combat against standard infantry. While he is one of the most expensive Guard characters, at only 130 pts, I still think he is a worthy inclusion in the army. If you take him as your warlord, he must take the Master of Command trait, giving him the ability to issue orders. This works really well with his re-roll bubble, allowing you to also gain re-rolls to wound on a nearby unit or additional shots.
His Commissar abilities have taken a bit of a hit with the changes to their rules, so he may actually be a bit of a liability in some situations, but should help keep your units in check a lot of the time.
The Lord Commissar is a decent combat character to help support your infantry units. Armed with a power sword and with 3 attacks, hitting on a 2+, he can provide some decent support to your army. The change to the morale modifying rules make him a bit less useful in games, but he can still potentially save you from casualties a lot of the time. I probably wouldn’t be fielding him much of the time, going for cheaper Commissars or another Company Commander.
Colonel “Iron Hand” Straken
Straken is actually a great combat support character for Catachan armies, able to deal a fair bit of damage in combat as well as boosting nearby Catachan units.
He has 4 attacks that hit on a 2+ at S7, -1AP and 2 damage each. He also gets to re-roll failed wounds on enemy monsters, making him quite potent at taking these on. He is also fairly durable, with T4, 5W, a 3+ save and 5+ invulnerable.
One of his big bonuses is that he gives all Catachan units within 6” +1 attacks in the fight phase. This is a great boost, doubling their attacks in most cases. This is even better when combined with a Priest for an extra attack to all units within 6” as well. This could actually combine to give quite a potent counter-assault unit if well equipped for combat.
A key unit for giving orders to the brilliant Militarum Tempestus units. This model gets to issue two orders to these units each turn and can also deep strike in with them on deployment, allowing them to target key enemy units. He has a fairly modest statline, but the real reason you will be taking him is for the orders ability to boost the Tempestus squads armed with special weapons.
This psyker knows one power from the Psykana discipline and Smite, but can only manifest one power per turn. There are a number of useful powers in the Psykana discipline, so I can see a use for using these over Smite in a lot of cases. At 40 pts each, it could be quite cheap to take a large number of these in your army to provide some very potent psychic support. If only they weren’t force to take the Force Stave, they would be even cheaper to field. Saying that, with 3 attacks, hitting on a 3+ at S5 and D3 wounds, he could be pretty good in combat if he was forced into it.
It could actually pay to have a Commissar around if you plan to field a few of these, as the For Your Own Good rule does quite well to protect nearby units from suffering mortal wounds if the Primaris Psyker does suffer a perils of the warp.
The Astra Militarum have some great HQ units for buffing nearby units, mostly in the form of orders. Of the HQ units available, I think I will be fielding the Company Commander, Tank Commander and Primaris Psyker most often. These provide some really nice boosts to the army, as well as some potent firepower for the Tank Commander, while still being cheap enough to field in high numbers.
The Special Characters also add some nice boosts to certain Regiments, so would be worthy of inclusion of you field these armies.