All the buttons mentioned in the title above have similar actions. They move the user back to the previous screen in mobile apps. What option is best to use consistently?
Back is very specific in mobile applications. You can go back in the browser to the last page visited. On Android you can move back in a stack or to a different stack and in iOS back is tied to the screen you came from, hence it can display its title. Remember that you can go "back" historically, "up" in a hierarchy or left in a series of pages where you can go back and forth freely (like a wizard).
Done is usually a confirmation and can have actions tied to it (save settings, send).
Cancel is dismissing whatever you did on this screen (restore settings to how you found them, deleting a draft).
To sum it all up with an example: Imagine you are opening a settings wizard. You can navigate back and forth in this wizard, change settings, adjust things, etc. Then you can cancel it and no settings will be saved. If you press done, all settings will be saved.