My feeling is that cover #5 is the most eye catching, and intriguing, and far more appropriate than the others, as it conveys so much symbolically. Here is my reasoning.
While the human "biological robot" images are very appropriate, I think they are more of an inside joke that Skeptiko regulars will appreciate straight away, but people unfamiliar with the content will miss the reference entirely. In fact, I suspect that anyone unfamiliar with the evolution of Skeptiko (and your personal journey through the world of establishment science) will not get what the image is referencing, and may simply see a silly cartoon on the cover. Only AFTER reading the book, or tuning into skeptiko will potential buyers of the book have that "I get it" moment, by which time you've lost them.
The image needs to speak to people who have no idea yet why mainstream science's touting of the biological robot story is absurd, and so I think that cover #5 of the guy running up a set of cogs will speak louder to the intrigued, but uninitiated potential readers.
The cogs I feel are a fairly universal symbol and appropriate metaphor for the Newtonian mechanistic science paradigm that most of us have grown up in, and that the average and reasonably educated man on the street will be very familiar with, and the image of the guy running across the cogs like a treadmill conveys symbolically the idea that this is going nowhere.
My first instinct was that I liked the biological robot imagery, but then I realised that it is only because I am very familiar with your hashing out of this absurdity, and using this term often in your podcasts. I realised then that potential readers of the book will have no idea what the image is attempting to convey, and I am certain most will totally not get it, and many will judge the book as something less than serious, from the cartoony and wacky image that they don't yet get. The cogs, and the guy running over them, most people with a little effort will get, and may then be intrigued to find out what you are saying about this in the book, whereupon, they will then discover the biological robots.
Phew, a little long winded, but there it is. Hope it helps somehow.
Soul