John Wesley Hardin being "the best irl" is an opinion.
He was known for being one of the more notably impulsive gunmen and like others of his type, seems to have made up many of his exploits, as they cannot only be verified but were also contradicted later by himself. But it doesn't totally come as a surprise to me that some fans would conflate that with being the best.
It's funny, too, considering Hardin is lampooned by a character like Jim Calloway in-game, and while similarly skilled in his own right, clearly had many fabricated exploits.