Using the Civil War as the criteria, i.e., accomplishments in fighting the war to a successful conclusion. seem, to me to be the most objective(reliable).
As many on this board know already, I have usually answered this particular question, by just noting that Lee's major achievements, were confined to a a small corner of the War, while achieving stalemates, while Grant was fighting through all the major dept's of the War from the Mississippi to Appomattox; against most of the most famous and experienced Generals of the confederacy, including Lee, with uniform success.
Granted Lee's acomplishments in one small part of the War was out of porportion to what could have been expected, he managed only to extend the war in Va. between the Rapidan and the Rappahanock, while the North under Grant, et. al., in the West was continuously rolling up the confederacy from the West to the East. It can be argued that Lee would have been even better on a larger stage, but, in fact we do not know because he didn't, while Grant did.