From a MH/W basis it is hard to beat 5970.
My rigs pull about 250W per card (memory underclocked to 160Mhz).
I run 3x5970 per rig.
Using usb key for OS, underclocked sempron, and MB w/ everything unecessary turned off the entire system pulls ~860W at the wall.
Rigs get ~2.2GH to 2.3GH on 860W or ~2.5 to 2.7 MH/W.
That's pretty close to my setup. I'm getting about 2.1GH on 850W with 3x5970, but my cooling is poor. 2.5 MH/W is pretty easy to reach with 5970s.