I have tried RetroArch, which is pretty good, but has a major flaw: there is now way to use the right joystick on a Xbox360 controller.
When I press the R2 button, it acts as if I pressed the C-Up. And you can't re-map it in the menu :(
The emulators by Robert Broglia have excellent game pad support. I use them with a wired 360 gamepad and a Moga Pro bluetooth gamepad. All his emulators can also be installed onto the Nexus Player through the play store website.
https://play.google.com/store/apps/developer?id=Robert+Broglia