Imperial Specials
Commisar Yarrick - Excellent if facing an army that induces lots of morale checks like chaos or tyranids. All your ig pass morale if they can see him. Just be savvy and protect him, especially if your opponent is keyed up as they will try to take him out. I usually get him to an imperial building and look upon the troops

Inquisitor - Same as yarrick but short range, ideal as a filler for an army, especially if you have a suicide squad so they can just keep going till done
Kreugers Heroes - not used them yet but can see benefits of being able to induce -1 to enemy morale checks and same price as a leman russ squadron. Thing is i don't play my armour up front assaulting, usually its tank hunting and supporting fire.
Capitol imperialis - never used as never seen any benefit. May have 6 void shields but lacks any ranged or good weaponary (cannon may have 4 shots but 25cm range!). Why have troops in it moving at max 10cm and will all die if its destroyed.
Leviathan - This i like and do use. Very good arnament 75cm ranges so can take on a titan, plus the doomsday cannon in direct fire can be pretty deadly. Only 4 void shields bit of a low. But the biggest factor is that you must not let it get destroyed as you will loose a turn with all your army on shoot in advance phase only as it is a top part of the command chain!! When i feild them i field more than one to try and stop this happening, believe me it has happened to me once and that was bad, one whole turn of my opponent having free reign.
Be warned!!
Commisars - The best free troops you can have. can corder detachments and keep them in chain of command. can be used as small fast seize troops, or as a great fast assault group. Always keep some handy for units that may wander out of command or if your opponent going to target your hq's. I always when deploying them think where shall i put them as so many, but by later in game i think where are they need one here and here.