Nvidia Shield... really... nothing else is as powerful as the Shield.

There are 2 other official devices on the mass market available at retail. Mi Box S and Jetstream neither are nearly as powerful.

For official AndroidTV stuff, you are limited as the only official devices are few.

If you're talking UI stuff being the only factor and are okay with your device running a bastardized version of Android TV, the Mecool KM9 is running the S905X2, which is a better CPU and GPU than any of the "official" devices. While it runs Android TV, it is not Google Certified and does not support Chromecast, and has some DRM issues.

with your requirements, youre looking for the Nvidia shield