That depends on your budget. In order of expense/performance
R7 260x - $100
R7 265 - $120
GTX 750ti - $140
R9 270 - $150
R9 270x - $170
R9 280 - $180
GTX 960 - $200
R9 280x - $220 (This is the one I would choose)
R9 290 - $280
R9 290x - $320 (I would chose the GTX 970 over this)
GTX 970 - $330 (Best card for the money right now, regardless of only having 3.5GB of usable memory)
GTX 980 - $530 (The best consumer gaming card there is right now)
GTX Titan Black -$? (Meant for rendering and workloads, but yolo)
Really you should upgrade your whole system. It's about that time. If you would like to start with your new graphics card that's cool. Here is a list of prospective new platforms you could jump to listed in order of...