(inspired by comments in the TO thread)

I say...

NFL: Terrell Owens, to pump up the fun quotient and get back to NFL controversies being about how TO is simultaneously one of the best players in the league and a horrible teammate.  (EDIT: RK84's right, forget TO and swap in Barry Sanders/Bo Jackson)

NBA: Michael Jordan, to put to rest all this nonsense about LeBron being the GOAT.

MLB: Pete Rose, as he would surely ignore the rule that you can't run catchers over anymore.