When it comes to the best performance-per-Watt, the GeForce GTX 970~980 was generally leading the pact.
If I were to build a SteamOS/Linux living room gaming PC today, I would likely settle for the GeForce GTX 960 or GeForce GTX 970 for a 1080p display. The GeForce GTX 960 can be found for around $200+ at Amazon while the GeForce GTX 970 is closer to $300. With the GeForce GTX 970 you can get high-end performance while still having nice options for small form factor PCs like this GeForce GTX 970 Mini ITX graphics card.
If you are a devout AMD user, you'll be best off waiting.... Either for the RadeonSI+AMDGPU open-source driver stack to support enough of OpenGL 4 to support all interesting Steam Linux games at decent frame-rates, a miracle to happen for the Catalyst OpenGL Linux driver, or if AMD is able to deliver a competitive Vulkan driver in a timely manner and game developers are quick to jump on this next-generation graphics API.
https://www.phoronix.com/scan.php?page=article&item=steamos-22-gpus&num=1
Interesting article here on how AMD and Nvidia compare on SteamOS/Linux - Well worth a look for those who are interested and plenty of game results.