We know the difference between a fact and opinion, as it is currently defined. But sometimes there are opinions that are so obvious that they may as well be facts. It is impossible to sustain an argument that John Gochnauer is a greater player than Honus Wagner. One would need to be rather anal about strictly adhering to the true definition of fact and opinion to say that this is entirely subjective, and cannot be classified as fact.
On the other hand, if I made the statement that Rogers Hornsby is a greater player than Joe Morgan, while I believe it to be true, I think that is a subjective choice. Even limiting it to just Hornsby supporters, most of us would agree that it is close enough to say that Hornsby over Morgan is an opinion, and not a fact as I am defining it here.
However, there are some people who rank Hornsby much higher than Morgan, and consider the choice to be so obvious that to them, it would be absurd to choose Morgan over Hornsby. So if I were to offer the statement that Hornsby was better than Morgan, they would say FACT.
And that brings us to this game. I will start off with a statement comparing either two players, teams, player/seasons, team/seasons, etc. The next person will say FACT if he believes it to be ridiculous to see things any other way, or opinion if he disagrees, or thinks the comparison is close enough to merit some thought. I cannot stress this enough. Even if, say, you have Gehrig ranked ahead of Musial, and are firmly set on that, if you can see any argument for Musial that doesn't venture into the realm of the absurd, you should put OPINION.
I guess that covers it. Remember, put FACT or OPINION in capital letters when responding. And your challenge to the next person should force them to think. I used an obvious example in the Wagner/Gochnauer comparison as to what would be defined as a fact. The choices we give should be a little harder to distinguish, but not so much so that we are always stating OPINION, given the criteria I have explained.