Babe Ruth has to be #1. Many considered him the best pitcher of all time until he became an outfielder. A couple of his World Series pitching records lasted for decades. He hit more homers than entire teams for a a season. Gretzsky never scored more goals than entire teams for a season.
It's unthinkable to win the 100/200 double in three straight Olympics. And Bolt didn't just win them. He won them easily.
For about fifteen to twenty years after he retired, Wilt had more 50 point games than everyone else in NBA history combined. He led the NBA in scoring, rebounding and assists. People complain about playing 36 mpg these days. He averaged 48.5 mpg one season. Hell at age 36, on two really bad knees, he averaged 43.2 mpg, shot 72.7% from the field and led the league in rebounding with 18.6/game.
As to the myth of not playing against any big guys, a perfect example is 1966-67, Wilt averaged 24.1/24.2/7.8 (when assists weren't hockey assists), shot 69.3% from the field and played 45.5 mpg. In 36 of the his 82 regular season games he played against Hall of Fame centers.