This was my comment for the primarily yellow cover with the picture of the divided face:
I think this is best because:
1. The image displays the central theme of human being versus automaton (the pseudo-scientific, materialist view) at a glance. It's also nice that the left/right divide of the face reflects the popular notions of left vs right hemisphere function. There's also the difference in colour, which reflects ying and yang. Like I said, a simple, bold image, but rich in symbolism
2. It's a striking, bold design and colour theme; aesthetically pleasing. Black, yellow and red are the colours of dangerous animals and this is aiming to be a provocative book that does what it says on the tin.
I don't think I consciously applied a rating: I may have missed doing that. If I did, maybe others will too. My rating would have been full marks for it, with no comments or rating on the others, which I don't think anywhere near as good.