The Big O had a really cool concept and featured some neat inspiration from Batman: The Animated Series...buuut, I'm not too sure I was into the few episodes I've seen all the same. I'll go with Mushi-shi.
My vote goes to Mushi-shi.
I have to give the nod to Mushishi definitely, I've actually never been very enthused to see Big O.
I'll give one to Big O
BIIIIG O Showtime!!