Google Cardboard is a specification, not a single device. The options can be kind of confusing, but my best experience has been with the Mattel ViewMaster VR. The ViewMaster VR fits small and large iPhones (including the 6+), has decent lenses, is "v2" (so uses a capacitive touch switch instead of the magnet), but also has a hole for the camera to see through for augmented reality experiences.
The only other "cardboard" I've tried was pretty crappy (by MINKANAK)—the lenses were subpar, and the phone didn't do a good job of staying in place. If you want to see a larger selection of cardboards, Google has a store.