The 6670, but really all of those are terrible for gaming. You might be able to play things like city builders and turn-based strategy games, maybe RPGs with very little real-time animation. But if you play a game with any significant live action, you'll be watching a slideshow.
If you want to play anything like a modern game, I'd say the bare minimum would be an r7 260x or equivalent, but try to go a tier or two higher if your budget allows:
http://www.tomshardware.com/reviews/gpu-hierarchy,4388.html