Regardless of whether it's a decathlon or a battle to the death on a 100-acre plot of land, I'm sticking with American football players. I'll take the likes of Derrick Henry, Montez Sweat, Chase Young, etc against your choice of futbol stars, marathoners, tennis players, etc. All day.
I'm not scrolling back to check, but rugby players are pretty fit.
The original post is titled "What sport has the best athletes?" I take that to mean on average, not to cherry pick one/three/five, individuals to represent a particular sport and claim victory. I referenced the Superstars early on (w/o crunching numbers) and if that's the yardstick then I guess three-peater Kyle Rote Jr and soccer win. Maybe there's a case for Bo/Lebron/Usain if projecting for the 21st century. On average, however, among "major" (read US) sports, no way it's baseball or football. Maybe reframe the question to take the population of folks paid to perform their craft, select 10% of them at random, and contemplate the hypothetical outcome of a Superstars-like team competition.