5830s are the most cost effective, but they are hard to find anymore. The 6870 is the next best, since it is only a little more and can go up to 330 mh/s. Only $160 at Newegg this weekend...
Only in shortterm view. 5830 is worst in Mhash/s/W. It is crippled 5870, so it has power draw of 5870, but only 2/3 output.