Night Lords!; In their presence the black legion turn white, the iron warriors rust themselves and the word bearers stammer.
The more blasphemous and decadent, the merrier! So from what we have to choose from, it's easily Word Bearers. Other than that, I'd also consider Emperor's Children. There's nothing like crucifying loyalist dogs or torturing/raping some innocent imperial citizens!
Iron Warriors all the way. In true ghost busters fashion: *When there's something strange -- In the citadel -- who you gonna call? Iron Warriors!*
I will join Word Bearers only because there are no Thousand Sons my first choice or World Eaters my second choice. So a more detailed resppnse the 4 core powers should have been represented along with Black Legion and Night Lords. Hopefully in the future i get to play a Thousand Son or a Berzerker of the World Eaters