either a Rx480 8GB or a GTX1060 6GB depending on what you find cheaper
according to au.pcpartpicker.com you can get a Rx480 with custom fan design for around 420$, a GTX1060 for 430$
if you look around a bit you'll find something for 400$
if you wanna save money, go for the 4GB (480) / 3 GB (1060) versions.