Definitely Kakuzu, i'd have to kill him 5 times before it was over; besides in his fuzzy stage (you know with the hairs coming out of his mouth and other place) he was DISGUSTING!!!!
I wouldn't wanna fight Madara or Sasori or Kisame because their techniques kill. If I had to choose I would fight either Zetsu or Itachi, Zetsu because he's not really a front line fighter and Itachi because he's not a bad guy