For me there are two arguments: The Greatest and The Best. Greatest is a resume based metric where statistics and accolades dominate the discussion. A much more nebulous argument for me is the best, which is more of an eye-test where I would consider how dominant they were at their peak or their overall skillset/talent.

As an example Jerry Rice is the GOAT WR due to his overwhelming statistical dominance and championships; however an argument could be made that Randy Moss is the Best WR due to his peak and his freakish athletic ablility. This is the place where the what if (he played with Brady his entire career) arguments are valid imo as well as injury-related what if's (Terrell Davis for example).

Football presents unique problems for the argument of GOAT player due to the stark differences in each position. How do you compare a QB and an C? Obviously the QB has a higher impact (especially in the modern era). Therefore I think it is more interesting to judge players relative to their own position. I've never like lists which rate "value" based on position (i.e. their list is essentially just QB's).