The same good and old rx 470, rx 480 that you can find, only eth is having a good return.
Pretty much what he said. Just make sure you buy the 8GB models or the 4GB will only run until Nov/Dec of this year. These cards are dirt cheap because there are tons of them out there and gamers typically want Nvidia. Most of the Nvidia stuff is a little more efficient and faster however even buying second hand is expensive.
If you ever tried selling any GPUs on Craiglists, you will find that when you list an Nvidia GPU, usually you get 3x the responces compared to say an AMD GPU. Hence there is less demand and prices are cheaper which is good for you. Just stick with those.
