I don't see him as a fighter with one loss to be honest. The second Greb fight seems to have been a Schmeling Sharkey II style robbery, and there are a couple of ND fights which he very arguably had the worst of. It should also be noted that the one unquestioned loss, was a pretty darn comprehensive one.
For a top boxer, never having tasted defeat is of course a bit special... but even if you change the Greb loss to a win, I don't think his overall record would look much better (if at all!) without the ND matches.
Tunney won the series over Greb. And even offered him another match, Greb refused! The " official " loss for Tunney came when he was a bit pre-prime, but he had to fight under harsh conditions with a cut eye and busted nose which happened in round one. I'm not sure exactly how the damage occurred, but there is such a thing as rising your stock in a loss. Tunney sucked it up and went the distance. The crowd gave him an ovation. Tunney is a marine. Very tough.
I don't really penalize guys for losing to Harry Greb so the loss doesn't really affect his standing, at least in my mind. That said, there is something a little magical about an unbeaten record. However the biggest thing Tunney could have done to boost his standing would have been to hang around and notch a few title defenses. That said, there weren't a ton of enticing options, hence the retirement move. I've got to think though, if Tunney had hung around and tossed Sharkey and a few other guys like dominos, he would be more prominently featured in discussions of the greats of that era. His biggest legacy though is probably his role as a sort of James Corbett 2.0, a highly intelligence fighter whose attention to the science of boxing and obsession preparation influenced future generations.