It's too difficult to cross-pollinate. I can't compare a shooting guard to a quarterback to a boxer.
So I'll categorize by sport.
NBA ... Jordan
NHL ... Orr
MLB ... Ruth
NFL ... Brady (I'm tempted to pick Payton because he could do so many things)
Soccer ... Pele
College football ... Tebow over Herschel
College basketball ... (tie) Maravich and Alcindor
Boxing ... Ali
Golf ... Nicklaus
Tennis ... Federer
Posting ... Clearly me
As I’ve gotten older, I’ve realized how difficult it is to make these comparisons. Part of the problem is that the games change over time making it difficult to compare guys that played the same position in the same sport in different eras. Take NFL QB for example. Nowadays, the defense can’t hit the QB like they could 30-40 years ago, nor can they jam the WRs like they could in years past. Therefore, QBs like Bradshaw, Staubach and Montana aren’t going to have the passing stats of a Brady or Brees, nor will they have the health and longevity that gives them more chances to win Super Bowls. Same with the NBA. No 3 point shot back in the day so lower career stats in points scored, triple/doubles, etc. Guys like Jordan didn’t get mugged as badly driving to the basket as guys from earlier generations and little nuances like push offs from shooting guards became more acceptable in recent times, etc. It’s just so difficult to crown the GOAT in many sports because of this.