6950 is the best miner. I use a pair of them unlocked to 6970 shaders and OC'd 920 core w/ +20% power and get ~750mh/s total. Not bad seeing as I built this as a gaming rig and just recently found BTC.
Is it the best though ? What was the cost for this?
2 x 5850's get just over 700HM/sec as well... at maybe 1/2 the cost!