Messi is unique.....best soccer player but not the best athlete in soccer.
If we are talking "most athletic", in the last decade I have it as a tossup between Lebron the Amish and Ronaldo. Both guys are so physically superior to their peers and both play in sports that incorporate all physical elements that I value when evaluating "greatest athlete"....speed, agility, strength, stamina, skill, mental ====>> dominance. These two tick all the boxes.......players like Brady, Woods, Phelps, as dominant as they are/were......are not complete athletes in my ranking system.
On the female side.....I've stood next to Serena before a US Open match....and I was scared. Because she could physically crush me and most of the male species, I give her Greatest Female Athlete designation and I quietly exit.