One of the quickest ways to start a baseball argument is to simply say, this guy was the greatest of all time. Football and basketball fans make that debate all of the time. And the opinions vary so much from state to state. Most pundits hand the crown to Michael Jordan, his six titles, and numerous MVP awards. Football is a little trickier since you need to add not only the greatest offensive player, but usually, they pick them by position, such as the greatest QB or linebacker.
I think in baseball, with all the changes in how players prepare, the rules, addition in the late ’40s of players of color, it is probably best to rate them by era.