I didn't like to use cases on my iPhones until the 6 series. Prior to the 6, iPhones were durable and sturdy. Not to mention their excellent design was too great to hide under a case.
My 6 and 6S both felt like really slippery, delicate devices that need to be babied, so I use the Apple Leather case. It also covers the ugly back of the phone so it actually looks better in a case than without.
Edit: I also never use screen protectors. My screens have never been scratched, although the my 6S has this weird line between the glass and the screen. You can't feel that scratch on the surface. It's quite odd.