When people is talking about the greatest athlete of all time I personally like to concentrate on athletics. Three best athletes are Carl Lewis, Paavo Nurmi and Jesse Owens.
I´m not sure whether Owens is the best or not but I think he was the most effective. His WR in long jump (813) lasted 25 years while Bob Beamon´s amazing 890 lasted "only" 23 years. He did six WRs in 45 minutes and the best of all, he won four gold medals in Berlin in 1936. If you know even a little about history you know why this is so interesting thing.
Perhaps Nurmi was even better but now he will be second. What Nurmi did in Paris in 1924 was something that hadn´t been done before and after 1924 there hasn´t been anything that is even close to it. Five gold metals in four days (and I´m talking about long distance running), two (1500 m and 5000 m) during a single hour. Between 1920-1928 nine gold metals (ties the record) and he couldn´t even run two competitions that he would have won because the other Finns also had to won something.
Lewis also won nine gold metals. He didn´t do anything that special but the competition was much harder than it used to be before WWII. Four consecutive gold metals in long jump is perhaps the best achievement.
The fourth could be Al Oerter and the fifth Sergei Bubka.
Also few athletes competing in winter events should be noticed.
The first is Eric Heiden in Lake Placid in 1980, five gold metals is an incredible achievement.
The second Björn Dählie, perhaps the best winter sport athlete overall but perhaps that´s the reason why he´s the second in my list.
The third is either the best ski jumper ever, Matti Nykänen or the greatest skiier that is perhaps Ingemar Stenmark or Herman Meier.