I'd go with Dark Eldar - I think that the new models are awesome (plus, some models don't even exist, meaning you have to make your own conversion; I actually saw some photos of a Venom that are full of win); furthermore they play differently from your chaos doods.
Sisters are a great army too,
imho, at least from a modelling pov: they've got Exorcists and all the cool gothic troops. However (I'm not up to date with the news so I could easily be wrong), wasn't the new
GK codex confirmed to be the only new army book related to the Inquisition? That would leave the Sororitas armies in a bad situation, lagging more and more behind everything else.
IG... For me, it depends. I love the Forgeworld models of the Death Korps like everyone else, and for regular
IG I'd try to make em like like
this. I don't particularly like catachans but I understand you can throw in all sorts of cool details, like cigars, shades, cowboy hats...
For the others, I've got nothing to say