I have read many times where people are actually bathing dogs in people shampoo. Dog's hair is totally different than human hair, so scientists have found just the right PH formula for dogs.
Anyway, the most gentle of all shampoos would be Dr. Bronners liquid castile soap that's gentle enought for new born babies, people, kids AND pets.
He even has eucalyptus and lavender oil. Those one repells fleas.
http://www.drbronner.com/drb_sai_liquids.html