I have the Ringke Fusion, and I absolutely love it. It adds a nice tactility that means the phone doesn't slip around when using it, comes with things to close up the headphone and microUSB ports for water resistance, and even comes with a cutout of the shape of the back so that you can make your own design for the case. It's great.

Nexus 6 Case, Ringke [Fusion] Clear PC Back TPU Bumper w/ Screen Protector [Drop Protection/Shock Absorption Technology][Attached Dust Cap] For Google Nexus 6 - Clear https://www.amazon.com/dp/B018UMKBFI/ref=cm_sw_r_other_apa_fmp8xbXYY6Z71