Mostly from a model standpoint, though a few rules and bit of fluff might have crept in.
AM (formerly IG):
probably the new Scion models, honestly.
Hmmm. Odd. I have 4k points of Daemons and I honestly can't say I'm impressed too much by their models. Odd that I'm just realizing this now. Fave models? Probably the OOP 4th ed Daemonettes--for the graceful, deadly aesthetic as much or more than just the "
boobies" reaction (though I'd be lying if I said that entirely weren't the case).
Probably the new Dark Vengeance kit, honestly. Terrible models to play with, but some of the best sculpting that most perfectly encapsulates CSM to have come out of GW. Helbrute, Chosen...
Incubi, unquestionably. I drool over those models frequently and have to bite back the urge to buy just one box for display purposes only, in spite of not having any of the army at all.
Probably Wraithguard, for their utterly alien aesthetic.
I have to go with Deathmarks. I suppose I'm weak for mono-eyeball cyclops models...
Big Mek with Shokk Attack Gun, I think. Just for the wacky, wild weirdness.
Space Marines (inc DA, SW, BA, GK):
Sternguard. Last edition's kit and this edition's kits have both been fantastic for this unit. Narrowly edge out the new Librarian model.
Hmmm. Great fan of the fluff of genesealers, but less so the minis for them. Probably the Trygon model, just for how damned impressive it looks.
Edit: forgot Sororitas
: Got to be Arco-Flagellants, more for the fluff than the models but the models aren't half shabby. In much the similar vein as the Penitent Engines, I suppose.