There might not be a best grade of loan. I can see where LC attempts to have their underwriting criteria and interest rates balanced to provide an even return to investors for any given grade. However if someone else could do better predictive analysis and even weed out 25% of the loans that will probably default or find a way to predict the ones that won't, then the lower grades offer more potential for higher overall return.
This seems to be what Bryce is doing with P2P picks and what Emmanuel is doing with the "Expected Return" feature of LendingRobot. Even Peter's relatively simple filters seem to weed out some of the higher risk loans. It remains to be seen how they perform over the full term of the loans, but I do believe their algorithms can lead to improved performance vs simply investing in random loans.