So far I'm really happy with the Nexus Player. I like that it gets Android TV apps (like Kodi and Moonlight for game streaming from my desktop) and also supports Google Cast. It also has a usb port that supports things like keyboards, ethernet, external disks, etc with a usb hub.
I also strongly considered the Nvidia Shield TV (also Android TV OS). It's $200 with a lot better hardware, getting good reviews, supports 4k, and apparently can decode H265 in hardware.