well, if you can afford it, the 280X. if you're looking to save a bit, there's some R9 280s (which are basically 7950 Boosts) for under $200. the 270X is viable if you just want to save a heckuva lot more.
ignore the 7950 unless it costs close to or under $200. otherwise, the GTX760 is a better option then.
http://pcpartpicker.com/parts/video-card/#qq=1&c=142,137,71,167,148&sort=a8