Best athletes rank them.
NFL
NBA
NHL
PGA
MLB
EPL
What is meant by "athlete"?
For me, it's about overall coordination, skill and fitness, which would tend to lend itself to multiple sports/competitions.
Ultramarathoners or triathletes ... obviously incredibly, incredibly fit ... but are they "athletes"?
To me, they're not. But based on traditional definitions, they are ... in fact, they're more within the definition of a true "athlete" than most participants of other sports. Track & field athletes were the first real examples of the term. So maybe I'm way off base. I see really fast people, or really strong people ... and they exhibit just these skills ... are they accomplished athletes? To me, you need to apply these skills toward some other end.
Take soccer players ... Fit, but they're friggin banned from using their arms (outside of throw-ins). I have my American bias, of course, where you see all the best athletes abandon soccer early on in youth, and go on to succeed in more traditional American sports. Only the kids without the coordination or size to compete in these other sports stick with soccer. I know, I know ... soccer fans will point out the exceptions ... but I'm talking about the general rule. It's just hard for me to picture a superior athlete not having premier (or, at the very least, demonstrable and necessary sufficient) hand-eye coordination.
Based on my own definition, I would think the NBA would house the best overall athletes, because basketball typically requires speed, agility, explosiveness, endurance and upper and lower body coordination. It doesn't require as much strength as some other sports ... and there's also the fact that height is REALLY meaningful, and sometimes overrides "athleticism" as being of primary importance ... and often excludes very skilled, very athletic people from competing at the highest levels.
NHL is too "niche" to really house the best athletes.
NFL is a mixed bag, as most of the positions are specialized. A WR is just SO different than a LT. Size matters, greatly. It's far more important than overall skill ... it's like height in hoops. But the biggest, strongest, fastest kids (combined) usually go on to play football.
MLB is niche as well. It takes far more coordination (hand-eye, especially), than any of the other major sports ... and you'll generally find that if you play baseball well, you can generally hold your own in most other sports (the same can't be said, to nearly the same degree, for other sports), but you don't have to be the biggest, fastest and strongest to play it ... so does that eliminate it from consideration?
It's also maybe worth noting that most sports are basically just variations of each other. Take soccer as the starting point. You run around a field kicking a ball toward a goal. What's lacrosse? The same, but, instead of kicking it, you're using a stick to run around with it and throw it toward the goal. Hockey? The same concept, except on ice with skates, and the object glides along the ice. Basketball? Basically the same, except you dribble the ball toward a goal above your head, that you "throw" the ball toward. Football? Well, that's more of a variant of the same general concept, with more pronounced stops and starts, and more varied skill sets in play.
Golf and baseball are the exceptions.