The Apple Leather Case. Very happy with it on my 6 Plus. Its the 2nd most expensive case I have ever gotten for a phone (I got the Fré for my 5 and didn't care much for it). This is the perfect case for me. It looks premium, feels premium, its thin enough that I can use my 6 Plus with one hand but thick enough that if I drop it the phone will be OK. Very happy with it.
When I had my 5 at this point (4 months in) I had a wide range of cases to choose from but I kinda stopped doing that. To me, it is pointless to keep swapping cases. When I got my 5s I just stuck with my Apple Leather Case in black (which I loved) until I got my 6 Plus, which shortly after getting it (less then a week) I got the Midnight Blue case.