Naked. I've had the Nexus 4 for almost three years, and it's been sans case the vast majority of the time. I've dropped it several times onto ceramic tile and concrete, and amazingly it doesn't have a single scratch, ding or crack. The 6P has an aluminum back, so it should be even tougher than the N4 with its glass back.
The N4 also is the last phone that I'll get a screen protector for. One reason is because I've never had a scratched protector. Another is because the 6P has 4th gen Gorilla Glass.