This is a fun topic to debate and one I'd be curious to get your thoughts on.
To run a sub 2:20 marathon is incredibly impressive. As guys out on the roads every day, we can acknowledge the blood, sweat, and ability it takes to get there. So to think that there are guys running BENEATH 2:10 is truly mind blowing.
Running is a full-body, weight-bearing exercise. For people who aren't used to it, it's THE toughest exercise to do. Running fast for x amount of hours and minutes requires incredible athleticism -- speed, stamina, strength (especially in the core), fluidity, etc. So I wonder how runners stack up in terms of athleticism compared to other sports.
Let's lay down some initial criteria here: Athleticism is not synonymous with skill. In this sense, Tiger Woods and Magnus Carlsen are not world-class athletes. For me, this is enough to disqualify soccer (European football) players as the "best" athletes in the world, as that is a game largely based on skill. So with that said, here are my picks for the best athletes on earth:
1. NFL Players - As one of the highest paying sports on earth, Americans are bred to play this game from an early age. NFL players are complete athletes, capable of repeated explosive movements over the course of 60-minute games. If you doubt how athletic these guys are, check out an NFL combine video on Youtube.
2. Boxers - Boxing is without a doubt a skill-based sport, but world-class boxers are unreal athletes. The sweet science is another full-body exercise, essentially requiring your arms to act as legs for twelve, 3-minute rounds (all while your legs are moving from side to side and in and out). As a former boxer and MMA guy myself, I equate boxing to running in many ways. Only with boxing, you are not only pushing your own body to the limits, you are also dealing with someone else in your face the entire time. MMA fighters can be incredible athletes too, but take it from someone who have fought in both sports -- boxing is another level of difficult. You can't change ranges (from stand up to clinch or to the ground) like you can in MMA, which allows you to use a different set of muscles, and boxing matches last longer.
3. Distance runners - For the reasons I listed above. It's also worth noting that cyclists, boxers, and world-class rowers have never run anything faster than a 2:29 marathon.
Honorable mentions - Gymnasts, power lifters and world's strongest men competitors, swimmers, and soccer players