Yes it is indeed close to a "futile witchhunt" regardless of whether some wish to admit it or not. Human nature does not change regarding a favourite, regardless of some minor counter argument to the contrary... we like best who we like best, whether it is a pretty girl, favourite boxer, or whatever. Human nature is a concrete constant regarding favourites! Boxing is no different today, or yesterday in that regard. Some may just have a greater volume than others over time regarding boxing.
A good historian will provide multiple views of the event (where available) and leave it to the reader to deside, whether he believes it or not. Adam Pollack's books are that way, for example. Say, I was browsing Kevin Smith's "The Sundowners" yesterday, and I read in chapter about Bobby Dobbs that he beat Joe Gans easily and gained a well deserved victory. You go to the local source, Brooklyn Daily Eagle, and read: What about the other point of view, what did Gans and his manager say on the fight? Alas, Kevin doesn't provide any, leaving the reader certain it was just like provided above. The Baltimore Sun: The victory doesn't sound as convincing anymore.
In the case on no film, it is best to rely on newspaper reads, careful study of the ring records, and filmed fights of opponents. Historians input mean a lot too. For example, there is no flim of John L Sullivan in the ring, but its safe to say he was the best or 2nd best heavy in his day, and was good enough to rate in the top 10 decades after he retired. Another example would be Greb. He beat many all time greats, and there is plenty of flim of who he defeated.
Again, that's why I brought up detractors of great fighters as well as what the competition said about great fighters, as a means to counter blind supporters and human nature. This discussion, as I take it, is to compare fighters from different era's, and most importantly to me anyhow, is how they would fair against one another. With that in mind, it is impossible to objectively & accurately predict who would win in many of the fantasy matches even with an abundance of film. Just look @ the Dempsey vs. Marciano posts that pop up and you get a serious division among posters as to who would win this one, and not very many who would see it as a pick em' fight. I myself am 100% absolutely sure that Rock would KO Dempsey in the late rounds though!!! I will bring up the following again and ask you a question based on the available information of the following fighters: "Did Joe Choynski punch harder than Chris Byrd?"
The key point with the ever popular subject that is Harry Greb is that we do have footage of the men he beat, so we can get an idea of his real ability. Pictures, newspaper reports and personal accounts can go a long way if you sample enough of them, then you can start to observe patterns in his fighting style. Go read fight reports of fighters you do have film of, a particular fight, and see how accurate they are - alas this is the manual labour of an historian, but it is the only way you can start to get piece of mind on these old timers.