Yeah, the first one is my favorite as well. Poor little girl, it hurts me to see little kid zombies. I'd still pop a cap in one to save my ass though. XD
It has to be image no6 for me. I just think the diagonal slice and the whole body falling apart into two looks awesome. It just shows that the additional budget increase in season 3 has gone to good use and hasn't be wasted. I liked the others, especially the governors daughter getting the sword stuck through the back of her head and out of mouth but no6 is just that little bit special.
I guess you could attribute my choice to the whole "theatrics" of the kill. I agree some may have been more visually effective, but for whatever reason, when I picture Michonne now, I just see her shoving the sword through the back of Penny's head. Haha.