"And the angels of darkness descended upon pinions of fire and light...the great and terrible dark angels."
I voted for IF, mostly because I have grown to respect Rogal Dorn based off the Horus Heresy books. He made the Planet of earth into a fortress that was extremely difficult for his brother Horus to crack. And only had one or two instances of actual anger, mostly involving Peturabo. In close second I would say Space wolves, for the same reason. The Dan Abnett book really had me see them in a different light than the Blackmane series led you to believe the wolves were like. (but Alas there can only be one choice). Besides, I too am a true fan of the Black Templars, and I can't be dissing High Marshal Sigismund's Alumni!
I have a very good idea which would satisfy the majority of the playerbase. It is simple...really. We give ultramarines the boot, and we add Salamanders and Imperial-fists instead.