Pierre Cardin Leather case
While the new OnePlus 3T doesn't have the beautiful Sandstone finish as earlier models, it does come as an official styleswap shell from OnePlus. However, not everyone wants wood backs or weird textures.
Instead, if you're looking for an elegant and premium real genuine leather case, this is one worth considering. Made by Pierre Cardin, it features real leather with beautiful hand-stitching too. They promise top tier craftsmanship, and it comes in three color variations. I've tried their cases on the Galaxy S7 Edge, and came away extremely impressed. This will turn the OnePlus 3 or 3T into a great looking device.