Most people seem to disslike ork and nids, this IMO isnt true.
Orks lack all kinds of long ranged weapon so, you have two free turns of shooting before they reach your lines. Once they charge they will obliterate the squads the engage, but the other squad can bring their flamers and flash light on them and your ork problem is over. This ofc isnt the case with speedfreaks, but u have tons of heavy weapons to shoot the vehicles and they have less ork.
Nids are the same as orks, with the exception of their increased speed (leaping hurts!) and big beasties, though concentrating your fire, starting with the first (fast) wave and slowly moving on to the slower/bigger things, only very few nids will realy your line. And when they do, just do the same as with the orks, use flamers!
Tua seem be to an odd choise aswell, though they might seem to have a better gunline, guards have enough options (light infantry/ drop troops) to out manouver them. Having lots of guards firing at 12" really kill tau, they simply dont have enough modells to take the casualties.
Another option that comes to mind is: space marines, their new flexability might seem like a bitt of an overkill. But marines simply lack moddels/shot to kill all your guards. But, watch your step. Marines might nog be the nightmare, they definatly come close .
Choas? the same as with the marines, to few modelles and the new combat rules really downgrade them as a threat.
Necron? They really are a pain in the ass with their WBB rule, this makes your flash light about useless, one kill every 120 shots orso and unless you kill the lord, your heavy weapons will just be half as effective. But, necron lack heavy weapons (guass has been downgraded to much) and they have virtually no blasts (though particle whip is a nuiance if you let their monolith get close. I'dd rate necron just about as high as marines, both anoying, but not a nightmare.
Eldar, their guardians barely match the lvl of guardsmen and their elite troops seem to be to few in number to hurt you, but dont underestimate them, those pointy-ear can still finish you off.
Dark eldar, now thats interesting. Dark eldar warrior are far better then normal guardsmen and just 8 points (!), have reall cheap heavy/special weapon options, can have some blasts and have formidable CC units. Their speed will be a problem, they can/ will obliterate an entire flank in just one turn (if u let them play the game the way they want). However, the ever greatest DE weakness, they are vuleranable, very vuleranable. Get the first turn, pick your targets right and they will be down to earth (I have experience with this

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This only leaves the inquisition.
Grey knights wont be that much of a problem, even less models then marines (unless ofc with inducted guard, but those tend to be the overpriced stormtroopers). But sisters of battle might be a bit more serieus trouble, they posses lots of flamers and have acces to guards to fill their ranks. Though their lack a long ranged AT makes their vuleranable to your tanks, if u dont lett them come to close, their shouldnt be a problem with those.
So, IMHO Space marines, necrons and Dark eldar are the biggest nightmares for an imperial commander. Though I did asume both players used a competative/balanced list and there by, this doesnt goes for people with armoured companies or if you fight with/against other fluffy or unbalnced/beardy armies.
Greetz,
Levitian