In terms of raw mining performance, probably the 7990 (though it's actually a dual-GPU card, and I wouldn't want a dual-GPU card for anything but mining, microstuttering can be a bitch in games for example). I think the 290x is the next one down, but it's also pretty expensive.

The 280x is the best value for money card for mining at the moment from what I've heard. It's up in the ~700KH/s range while being significantly cheaper than the 290x.