Thinking about turning a old PC into a couch gaming machine using SteamOS. What would be the best GPU for around $250?

I have had my old gaming computer sitting in a corner for a while, it is a build from mid 2014. i5-4670k and 16gb of ddr3. Currently doesn't have a GPU, but I am aware that linux/steamOS is better suited to AMD cards, so my question is what is the best GPU option for my use case for around $250? Budget is flexible though.