One issue I have with these debates is the inevitable 'if (insert name of older athlete) played today, he could not compete because he would be (insert objection: not tall/strong/fast/quick/smart enough).
What those arguments miss is if yesterday's athlete had all of today's tools in their arsenal, they would be that much taller/stronger/faster/quicker/smarter than they were back in the day.
The best way to make a 'best ever' comparison is to avoid comparing athletes from different eras. Instead, compare them against their peers.
Taking football out of the equation how much better was Bobby Orr against his peers, versus Wayne Gretzky against his peers, versus Gordie Howe and Bobby Hull against their peers, etc.
If one is going to use the 'today's athletes are bigger/faster/stronger' argument, then by that logic then whomever is playing at the present moment is the greatest ever. Five years from now those athletes will be the greatest ever, as will those in ten years, twenty years, and so on. In other words the 'greatest ever' is no more than last year's all star team.
Sorry, but to me 'greatest ever' is far more than that.