I have to say that my favorite October MC is the Shadow Spirit. When I first got one, it was the perfect little companion to any of my black/violet/white avatars; The BVW theme was my favorite one for a very long time, so I was using it constantly in one form or another. Even removing the black, it worked - Because Gaia was so kind to give us the little Hitodama that many of us petitioned for...And they were violet!
That's got to be the biggest determining factor for me, that petition I signed ages ago. I don't know if Gaia really did listen to us, or if they'd been planning it all along...But that one little item made me feel important when it came out. I felt like our voices had been heard when we got our hitodama!