I would most enjoy killing traitor primarchs, but since they most likely wont be featured, I guess I will just have to settle with heretics. But killing some tau would be fun, tau or necrons are a close second. (for obvious reason seeing that I am a Black Templar that loves juice melee.)
I will enjoy killing Ultramarines for they are the one who stood before the great holy city and watch us as we were humiliated by Emperor.
a Primarch would rip your head off without even slowing down. That would be like setting a Space Wolf on a Children's Hospital. No contest. Grey Knights may stand a chance, but they were designed to stand in the breach against the worst of the worst.