I had a Apple Leather case on my 6 plus, along with a tempered glass screen protector. Two days ago I shed the screen protector and a day after that the case came off. Phone is absolutely amazing with how thin and smooth the curved glass feel.
A lot of people talk about the risk of using your iPhone naked and how likely they are to drop/break it. For me the downside of not using the phone the way the 6 plus was designed to be used outweighed the risk.
Either way, the Apple Leather case is the one I'd use if I was to slap a case on.