Best Mhash will come from a 6990x2, but as the above poster said, you need to consider a lot more than just that.  The best mhash/watt is from the 5830.  I chose the 6870 because I got a good price and I thought it was a good mhash/$.  If you re severely limited with space then you go for a more powerful (and expensive) care. If you have plenty of space and slots, then you try to go for the best mhash/$. 

The 5800's and 6800's are pretty good sweet spots for all of those statistics.