What part of the body should be washed first?

For best hygiene it is best to wash from head to toe. Hair first, feet last.

BS. I like to start by washing the dirtiest places first, you know where I mean, and wash your deodorant off so your body has a chance to sweat out some toxins while your in the shower, then the whole body, then repeat the dirty spots, don't forget to scrub your back, and finish off by rinsing in slightly colder water.