Some sports have a clear hierarchy of leagues and are statistically dominated by an elite group which makes placing their achievements easier - I am thinking of Wayne Gretzky - but it's probably applicable to the North American sports. Then you factor in how recently these achievements took place, because some sports have made such advancements in training, technique and equipment as to render comparison between eras almost impossible.
Then, the sports global popularity has to be a factor, simply because the more universal a sport the more impressive domination of it becomes.
To make my criteria I am looking for someone who has achieved global recognition in a sport that is accessible to everyone regardless of location or cost.
They should also be free of any nefarious suspicion.
After careful consideration (by which I mean inserting a comedy answer here) I am going for Haile Gebrselassie.
Set 27 world records during his career, beat his own marathon record at the age of 35, now grows coffee in his native Ethiopia.
Legend. You've heard of him, you're not sure why, but he's the pinnacle of what humans evolved to do; running at distance to exhaust prey to a state of collapse.