ATI ones.
6990 is obviously the fastest - but expensive.
All depends how much you have to spend, whether you care about power consumption, fan noise, heat generation, playing games, selling the cards on afterwards blah blah.
If you just want to go mining, mid-range ATI is probably your best bet.
Easiest thing would have been just for me to point you here:
https://en.bitcoin.it/wiki/Mining_hardware_comparison