The new Amazon Fire box looks like a winner. Here is the catch, it too like ATV is somewhat biased towards favour of the maker on its menu screens. ATV is heavy towards Apple purchase and rentals while Amazon towards its own store. However, I could live with this considering its a more capable player (meaning power and options compared to ATV).
Of late, I have been playing with the Nvidia Shield TV and if I had to rate strictly upon potential it would be
Nvidia SHield TV
Amazon's new Fire box
ATV (any incarnation)
My take is rather simple - what do you want out of a set top box? If you are fully invested in Apple's offerings of movies and rentals then most would be very happy with the latest ATV. If you are into side loaded services and games, then both the Amazon and Nvidia offerings are the way to go. In my world, I am staying with the Nvidia as it makes an excellent media player for my own media (not requiring conversion to another format like what is needed for ATV) while I will in October be setting up Amazon's Fire for a family member who is interesting in Amazon Prime, Netflix, and home media files to be played through Plex or later Kodi. (I prefer Kodi for my purposes).
In the end, there is something for everyone. I'll skip over the usual 4k debates as I don't have 4k and would much rather invest in a top notch device such as a yet to come to market Oppo Disc player that also does streaming services and my own media files.