Trick question. Shinobu is perfect throughout.
Please give me a better image of green dress bat if you have one, it's my number one bat.
I also like teenage bat, I wish she would stay in that form for an arc.
Cat > Snake > Snail > Doll > Crab > Vamp > Monkey
All of them, as they are Shinobu
This really is the key problem here.
I was going to guess because their hair style change and he didn't want to be divisive picking one, but crab cut her hair and he clearly preferred the long hair so he could have just gone with their Nisemono looks, which I (and I believe many others) found to be better looking than any other renditions of them anyway.