My favorite regiments are:
Gaunt's Ghosts, because of the characters and the fact that they refuse to die. I also like the whole we're the only thing that's left from all three intakes and how they all bond together. In Armour of Contempt, Ezra describes how they all have different scents and how they all still get along even though in his culture it would never happen.
Elysian Drop Troop, because they, like the Ghosts, take a practical approach to war and it's not just: Two men! One gets a rifle and shoots the rifle! When the man with the rifle dies, the second man picks up the rifle and shoots it!
Tlaloc Stormchasers, because they are my own invention. They are also only storm troopers or storm troopers in training and so they don't have big armies and are very efficient at their work, kind of like weak SM ;)
Preatorians, Death Korps and Valhallans. However if I get back into 40k my next project would be a Feudal World Regiment. I reckon I could mix up Cadians with Empire State troops to create the desired effect, and Rough Riders would have to feature, would love Pistoliers with Laspistols and some armoured with Cadian flak armour. Could be fun, though no doubt greenstuff would abound!
"Your eternal soul may belong to the Emperor, but your worthless hide belongs to me."
Gunnery Lt. Schenn (Navy), aboard the 'Manifest Destiny'
The Royal Volpone "Blue Bloods" Loyal to the Throne, and tough to kill!
and the 1001st Volscian (gotta love the idea of hive ganger regiments)
I'm a fan of the Cadians, Death Korps, Catatachan, and the Elysian Drop troops.
Of course, the Valhallans are pretty great too, especially Commander Chenkov. He so perfectly exemplifies the Imperium's attitude that Guardsmen are about as valuable as the las guns they're carrying…actually, scratch that. Guardsmen are less valuable than las guns. You can recover a las gun from a corpse and pass it on to the new recruits, can't do much of anything with a corpse. The Valhallans put the grim in grim dark.
Chenkov really missed his calling. I could definitely have seen him as a commissar.
Go to war? With a lasgun? What the f@ck am I guna kill with a flash light?!