I'm finally at the point in my VC campaign where I can afford one (1) army of whatever I want. How would you build a VC army to fight Empire, versus what would you add/subtract to fight Chaos? Or Greenskins or Dwarfs?

I was led to this question by trying to decide between Grave Guard or Cairn Wraith, and Black Knight or Hex Wraith. They are similar enough to make me wonder what would be best for any given enemy.