Otterbox. Best case you can buy. I have dropped my iPhone on a cement floor with an Otterbox and not a scratch. They are not cheap, but well worth the added expense in avoiding damage to your new $400 device. Otterbox has been in business for a long time making custom cases for expensive devices. The recommended model uses a plastic case covered with a rubber skin. It comes with a screen protector and don’t worry, your iPhone or Droid will work with the screen protector.
Now the problem with Otterbox is that normal custom mounts will not work with the larger case surrounding your device. A spring loaded cradle can be used to tightly (and we mean tightly) grasp your iPhone or Droid and keep it in place while riding. As we all know, GPS capabilities are excellent on smartphones and somehow makes us feel so much better about plunking all that money down for that new expensive phone!