These three are all great. I voted Massacre Girl because she will likely kill most of the board, however I currently run Massacre Wurm and love it. We have a few token decks running around and Massacre Wurm does work. Kagemaro is most flexible, but can also backfire if your hand is low and you need to kill something larger. Massacre Wurm has the same problem, but you know what your getting up front.
For non-black, False Prophet is great in a deck with enough sac outlets. Instant Speed exile and you can sacrifice your things before trigger.