Big time sports like football, baseball, and hockey are harder to analyze who the best was because there are a lot more positions which makes it harder to evaluate who had the greatest impact of them all.
I believe Tom Brady is the best quarterback of all time, but does that mean he's the best football player of all time? It's hard to compare his impact to Jerry Rice or Lawrence Taylor, two players who are also considered the best to ever play but at different positions.
Babe Ruth is considered the greatest hitter of all time, but did he have a bigger impact on baseball then say Nolan Ryan?
Did Wayne Gretzky have a bigger impact on hockey then Martin Brodeur?
Note: I'm not as passionate about baseball or hockey as I am about basketball and football so I'm just naming names. Hopefully you guys are catching my drift.
With basketball, the players play on both sides, so it's easier to evaluate who the best was even if there are five positions in basketball. Comparing Michael Jordan and Lebron James may be tough because they are different players, but we atleast get to see how they played on both sides of the court.